drm/msm/dpu: Remove unnecessary NULL checks

drm_device.dev_private is set to a non-NULL msm_drm_private
struct in msm_drm_init. Successful initialization of msm means
that dev_private is non-NULL so there is no need to check it
everywhere.

Signed-off-by: Drew Davenport <ddavenport@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>