drm/msm: validate display and event threads

While creating display and event threads per crtc, validate
them before setting their priorities.

changes in v2:
	- use dev_warn (Abhinav Kumar)
changes in v3:
	- fix compilation error
changes in v4:
	- Remove Change-Id (Sean Paul)
	- Keep logging within 80 char limit (Sean Paul)

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>