drm/msm: mdp4: Fix drm_framebuffer dereference crash

mdp4_get_frame_format() can dereference a drm_framebuffer when it's NULL.
Call it in mdp4_plane_mode_set only when we know fb is non-NULL.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>