drm/vmwgfx: Try to fix plane clipping

drm/vmwgfx: Try to fix plane clipping

Try to fix the code to actually clip the plane to the crtc bounds
instead of the user provided crtc coordinates (which would be a no-op
since those are exactly the coordinates before clipping).

Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101182920.14386-4-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>