video: mdss: Allow MMB alloc for video layers

 * During dynamic resolution change, more MMBs may be needed
   on a pipe in order to display a larger image.
 * Since we can be reasonably sure that the pipe will be reset
   when the video is done, there is no risk of exhausing MMBs.
 * Allow alloc in this case to avoid fallback.

Change-Id: I8ffc7d74b868defb5d76e7753204d8777f880d52
(cherry picked from commit c8bfd82e9b4efa12b80825639a02b0812bb5aa10)