drm/bridge: cdns-dsi: Use video mode and clean up cdns_dsi_mode2cfg()

drm/bridge: cdns-dsi: Use video mode and clean up cdns_dsi_mode2cfg()

The driver does all the calculations and programming with video timings
(hftp, hbp, etc.) instead of the modeline values (hsync_start, ...).
Thus it makes sense to use struct videomode instead of struct
drm_display_mode internally.

Switch to videomode and do some cleanups in cdns_dsi_mode2cfg() along
the way.

Tested-by: Parth Pancholi <parth.pancholi@toradex.com>
Reviewed-by: Aradhya Bhatia <aradhya.bhatia@linux.dev>
Tested-by: Jayesh Choudhary <j-choudhary@ti.com>
Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
Link: https://lore.kernel.org/r/20250723-cdns-dsi-impro-v5-12-e61cc06074c2@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>