drm/msm/sde: avoid duplicating plane states in crtc atomic_check

During crtc atomic check phase, plane states are retrieved with
drm_atomic_get_plane_state api, which internally duplicates the
plane state. This is unnecessary as the plane states are retrieved
only for roi validation purposes. Use proper api to avoid it.

Change-Id: Ia3ef4aec77ca3ca6f537384d72933904716993e9
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>