drm/mediatek: set mt8173 dithering function

Some panels only accept bpc (bit per color) 6-bit.
But, the default bpc in mt8173 display data path is 8-bit.
If we didn't enable dithering function to convert bpc,
display cannot show the smooth grayscale image.

In mt8173, the dithering function in OD (OverDrive) and
GAMMA module, we have to config them with
connector->display_mode.bpc when CRTC initial.

1. Clear the default value at *_DITHER_5 and *_DITHER_7 register.
2. Calculate the LSB_ERR_SHIFT bits and ADD_LSHIFT bits two values.
i.e. Input bpc of OD is 10 bits, we assume the bpc of panel is 6-bit,
so, we need to set 4-bit to LSB_ERR_SHIFT and ADD_LSHIFT bits respectively.
3. Then, set the OD or GAMMA to dithering mode depends on path-1 or path-2.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>