drm/vmwgfx: Replace vram_size with prim_bb_mem for calculation of max resolution

In the future, Scanout buffers need not be backed by VRAM and
the two definitions will differ.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>