drm/mediatek: De-commonize disp_aal/disp_gamma gamma_set functions

drm/mediatek: De-commonize disp_aal/disp_gamma gamma_set functions

In preparation for adding a 12-bits gamma support for the DISP_GAMMA
IP, remove the mtk_gamma_set_common() function and move the relevant
bits in mtk_gamma_set() for DISP_GAMMA and mtk_aal_gamma_set() for
DISP_AAL: since the latter has no more support for gamma manipulation
(being moved to a different IP) in newer revisions, those functions
are about to diverge and it makes no sense to keep a common one (with
all the complications of passing common data and making exclusions
for device driver data) for just a few bits.

This commit brings no functional changes.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231012095736.100784-9-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>