drm/msm/sde: size check fix for NV12 buffer

Currently for NV12 linear/compressed format if the same gem
object assigned to both planes, total size becomes twice more
than actually allocated. In that case kernel cannot detect
case where meta data planes not allocated for NV12 compressed
buffer and smmu fault happens. Current patch sums sizes only
for different gem objects allowing kernel to detect insufficient
memory allocation for NV12 case.

Change-Id: I0d9f49b8b310f0dff1fb787b4ba821a6d4a68140
Signed-off-by: Alexander Beykun <abeykun@codeaurora.org>