drm/mediatek: mtk_dpi: Add support for Pattern Generator in debugfs

This IP includes a Pattern Generator which is useful for debugging
and testing purposes: add the relevant register and bits to the
mtk_dpi_regs.h header, and implement support for it in mtk_dpi.

Adding this required to introduce a .debugfs_init() callback for
the DPI bridge, which creates a "dpi_test_pattern" file in the
directory of the appropriate connector.

The pattern generator can generate various internal patterns and
this submission includes support for:
 - 256 or 1024 shades of gray in a Vertical or Horizontal Pattern
 - Vertical Color Bars
 - Frame border
 - Dot Moire

This generator also supports filling the entire screen with one
custom color, but support for that is not included in this commit.

Enabling and disabling this generator can be done by sending a
string to the dpi_test_pattern debugfs file; the pattern is
expected to be formatted as follows:

         <enable (1) or disable (0)> <pattern number>

where the pattern number can be a number from 0 to 7, excluding 5.

Of course 5 is excluded because that activates custom color fill
which, again, is not supported in this commit.

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/20250217154836.108895-3-angelogioacchino.delregno@collabora.com/


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