drm: sde: Check commit's validity when starting splash handoff

User maybe send null commit with invalid frame buffer
to kernel, so early splash's handoff should wait for
one valid commit coming with active fb.

CRs-Fixed: 2225630
Change-Id: I5ef0809791d697be264787ebe7a6ec58704cb310
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>