drm/mediatek: change the dsi phytiming calculate method

Change the method of frame rate calc which can get more accurate
frame rate.

data rate = pixel_clock * bit_per_pixel / lanes
Adjust hfp_wc to adapt the additional phy_data

if MIPI_DSI_MODE_VIDEO_BURST
	hfp_wc = hfp * bpp - data_phy_cycles * lanes - 12 - 6;
else
	hfp_wc = hfp * bpp - data_phy_cycles * lanes - 12;

Note:
//(2: 1 for sync, 1 for phy idle)
data_phy_cycles = T_hs_exit + T_lpx + T_hs_prepare + T_hs_zero + 2;

bpp: bit per pixel

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Tested-by: Ryan Case <ryandcase@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>