Correct the allocation size for ssim_vars

Ssim_vars is used to accumulate stats based 4x4 pixel blocks, this
commit changes the allocations size to be based on mi_rows and mi_cols
to avoid out-of-bound memory access for larger size videos. The hard
coded 720x480 can only work for image size up to 2880x1920.

Change-Id: Id9d07f3f777385b448ac88a6034b7472e4cf3c79