drm/mediatek: dsi: Fix DSI RGB666 formats and definitions

The register bits definitions for RGB666 formats are wrong in multiple
ways: first, in the DSI_PS_SEL bits region, the Packed 18-bits RGB666
format is selected with bit 1, while the Loosely Packed one is bit 2,
and second - the definition name "LOOSELY_PS_18BIT_RGB666" is wrong
because the loosely packed format is 24 bits instead!

Either way, functions mtk_dsi_ps_control_vact() and mtk_dsi_ps_control()
do not even agree on the DSI_PS_SEL bit to set in DSI_PSCTRL: one sets
loosely packed (24) on RGB666, the other sets packed (18), and the other
way around for RGB666_PACKED.

Fixing this entire stack of issues is done in one go:
 - Use the correct bit for the Loosely Packed RGB666 definition
 - Rename LOOSELY_PS_18BIT_RGB666 to LOOSELY_PS_24BIT_RGB666
 - Change ps_bpp_mode in mtk_dsi_ps_control_vact() to set:
    - Loosely Packed, 24-bits for MIPI_DSI_FMT_RGB666
    - Packed, 18-bits for MIPI_DSI_FMT_RGB666_PACKED

Fixes: 2e54c14e ("drm/mediatek: Add DSI sub driver")
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240215085316.56835-3-angelogioacchino.delregno@collabora.com/


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