drm/msm/dp: reset drm_dev to NULL at dp_display_unbind()

drm/msm/dp: reset drm_dev to NULL at dp_display_unbind()

During msm initialize phase, dp_display_unbind() will be called to undo
initializations had been done by dp_display_bind() previously if there is
error happen at msm_drm_bind. Under this kind of circumstance, drm_device
may not be populated completed which causes system crash at drm_dev_dbg().
This patch reset drm_dev to NULL so that following drm_dev_dbg() will not
refer to any internal fields of drm_device to prevent system from crashing.
Below are panic stack trace,

[   53.584904] Unable to handle kernel paging request at virtual address 0000000070018001
.
[   53.702212] Hardware name: Qualcomm Technologies, Inc. sc7280 CRD platform (rev5+) (DT)
[   53.710445] pstate: 20400009 (nzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   53.717596] pc : string_nocheck+0x1c/0x64
[   53.721738] lr : string+0x54/0x60
[   53.725162] sp : ffffffc013d6b650
[   53.728590] pmr_save: 000000e0
[   53.731743] x29: ffffffc013d6b650 x28: 0000000000000002 x27: 0000000000ffffff
[   53.739083] x26: ffffffc013d6b710 x25: ffffffd07a066ae0 x24: ffffffd07a419f97
[   53.746420] x23: ffffffd07a419f99 x22: ffffff81fef360d4 x21: ffffff81fef364d4
[   53.753760] x20: ffffffc013d6b6f8 x19: ffffffd07a06683c x18: 0000000000000000
[   53.761093] x17: 4020386678302f30 x16: 00000000000000b0 x15: ffffffd0797523c8
[   53.768429] x14: 0000000000000004 x13: ffff0000ffffff00 x12: ffffffd07a066b2c
[   53.775780] x11: 0000000000000000 x10: 000000000000013c x9 : 0000000000000000
[   53.783117] x8 : ffffff81fef364d4 x7 : 0000000000000000 x6 : 0000000000000000
[   53.790445] x5 : 0000000000000000 x4 : ffff0a00ffffff04 x3 : ffff0a00ffffff04
[   53.797783] x2 : 0000000070018001 x1 : ffffffffffffffff x0 : ffffff81fef360d4
[   53.805136] Call trace:
[   53.807667]  string_nocheck+0x1c/0x64
[   53.811439]  string+0x54/0x60
[   53.814498]  vsnprintf+0x374/0x53c
[   53.818009]  pointer+0x3dc/0x40c
[   53.821340]  vsnprintf+0x398/0x53c
[   53.824854]  vscnprintf+0x3c/0x88
[   53.828274]  __trace_array_vprintk+0xcc/0x2d4
[   53.832768]  trace_array_printk+0x8c/0xb4
[   53.836900]  drm_trace_printf+0x74/0x9c
[   53.840875]  drm_dev_dbg+0xfc/0x1b8
[   53.844480]  dp_pm_suspend+0x70/0xf8
[   53.848164]  dpm_run_callback+0x60/0x1a0
[   53.852222]  __device_suspend+0x304/0x3f4
[   53.856363]  dpm_suspend+0xf8/0x3a8
[   53.859959]  dpm_suspend_start+0x8c/0xc0

Fixes: 570d3e5d28db ("drm/msm/dp: stop event kernel thread when DP unbind")
Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Patchwork: https://patchwork.freedesktop.org/patch/490756/
Link: https://lore.kernel.org/r/1655927731-22396-1-git-send-email-quic_khsieh@quicinc.com
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>