drm/mediatek: Only touch DISP_REG_OVL_PITCH_MSB if AFBC is supported

Touching DISP_REG_OVL_PITCH_MSB leads to video overlay on MT2701, MT7623N
and probably other older SoCs being broken.

Move setting up AFBC layer configuration into a separate function only
being called on hardware which actually supports AFBC which restores the
behavior as it was before commit c410fa9b ("drm/mediatek: Add AFBC
support to Mediatek DRM driver") on non-AFBC hardware.

Fixes: c410fa9b ("drm/mediatek: Add AFBC support to Mediatek DRM driver")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/c7fbd3c3e633c0b7dd6d1cd78ccbdded31e1ca0f.1734397800.git.daniel@makrotopia.org/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>