drm/vmwgfx: Filter out modes those cannot be supported by the current VRAM size.

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>