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

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

commit 9a72384d86b26cb8a2b25106677e1197f606668f upstream.

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>
[bwh: Backported to 3.2: drop changes for dev_priv->prim_bb_mem]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>