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,