resize-refactor: Check pixel dim instead of mb dim

Changes av1_scale_if_required to check if a resize is needed using the
width and height from cm directly rather than cm's mi_cols and mi_rows
times MI_SIZE.

This is simpler, and won't have issues with sizes that aren't divisible
by MI_SIZE.

Change-Id: Icbf34252a62436bb9bb0f83fa5fd688f7c5ae256
1 file changed