drm/vmwgfx: Fix corner case screen target management

drm/vmwgfx: Fix corner case screen target management

When the surface backing a framebuffer doesn't match the framebuffer's
dimensions, the screen target code would test the framebuffer dimensions
rather than the surface dimensions when deciding whether to bind the
surface as a screen target directly. This causes a screen target -
surface dimension mismatch and a subsequent device error.

Fix this by testing against the surface dimension.

v2: Fix review comments by Sinclair Yeh.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Cc: <stable@vger.kernel.org>