drm/msm/dpu: Only create debugfs for PRIMARY minor

drm/msm/dpu: Only create debugfs for PRIMARY minor

dpu_kms_debugfs_init() is invoked for each minor being registered. Most
of the files created are unrelated to the minor, so there's no reason to
present them per minor.
The exception to this is the DisplayPort code, which ends up invoking
dp_debug_get() for each minor, each time associate the allocated object
with dp->debug.

As such dp_debug will create debugfs files in both the PRIMARY and the
RENDER minor's debugfs directory, but only the last reference will be
remembered.

The only use of this reference today is in the cleanup path in
dp_display_deinit_sub_modules() and the dp_debug_private object does
outlive the debugfs entries in either case, so there doesn't seem to be
any adverse effects of this, but per the code the current behavior is
unexpected, so change it to only create debugfs files for the PRIMARY
minor.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
[DB: slightly change description and in-patch comment]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://lore.kernel.org/r/20220212003811.1818774-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>