v3d: Fix setup of the VCM cache size.

There were two bugs working together to make things mostly work: I wasn't
dividing the VPM output size available by the size of a batch (vertex),
but I also had the size of the VPM reduced by a factor of 8.

Fixes dEQP-GLES3.functional.vertex_array_objects.all_attributes and it
seems also my intermittent varying failures.

Fixes: 1561e498 ("v3d: Emit the VCM_CACHE_SIZE packet.")
(cherry picked from commit a91b158b)