soc: mediatek: mtk-mmsys: Fix MT8188 VDO1 DPI1 output selection

soc: mediatek: mtk-mmsys: Fix MT8188 VDO1 DPI1 output selection

The VDO1_MERGE4 hardware (merge5 software component) should be
set to enable output to DPI1_SEL by setting BIT(2) but, despite
the intention being exactly that, this won't work because the
declared register mask is wrong as it is set as GENMASK(1, 0).

Register MERGE4_MOUT_EN in VDO1 has four used bits [3, 0] so
fix the mask to reflect that.
That, in turn, allows the mmsys driver to actually set BIT(2)
in this register, fixing the MERGE4 output to DPI1 selection.

Fixes: c0349314d5a0 ("soc: mediatek: Support MT8188 VDOSYS1 in mtk-mmsys")
Link: https://lore.kernel.org/r/20250212100012.33001-3-angelogioacchino.delregno@collabora.com
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>