MLK-22599 drm/imx: dpu: kms: Support full screen CRTC background

MLK-22599 drm/imx: dpu: kms: Support full screen CRTC background

The CRTC background should be full screen instead of partial
screen, because the DRM core is likely to add configurable
background color support in the future.  We may cover the full
screen with ConstFrame0/1, upon which builds planes.  With this,
it is easier to compute each plane's layer offset vs CRTC start
point and all ConstFrame units can be controlled by CRTC.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
(cherry picked from commit ba18a9874cf010032413ca70f9b358399a143037)