drm/msm/dsi: fix bits_per_pclk

drm/msm/dsi: fix bits_per_pclk

mipi_dsi_pixel_format_to_bpp return dst bpp not src bpp, dst bpp may
not be the uncompressed data size. use src bpc * 3 to get src bpp,
this aligns with pclk rate calculation.

Fixes: ac47870fd795 ("drm/msm/dsi: fix hdisplay calculation when programming dsi registers")
Signed-off-by: Pengyu Luo <mitltlatltl@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/709916/
Link: https://lore.kernel.org/r/20260307111250.105772-1-mitltlatltl@gmail.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>