drm/msm: Add pm_suspend/resume callbacks to msm_kms

Used by the dpu driver for custom suspend/resume.

Changes in v3:
- None

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
[seanpaul split this out of the megapatch]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>