When screen objects are enabled, the bpp is assumed to be 32, otherwise
it is set to 16.

v2:
* Use u32 instead of u64 for assumed_bpp.
* Fixed mechanism to check for screen objects
* Limit the back buffer size to VRAM.
Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Cc: <stable@vger.kernel.org>