drm/modes: Only consider bpp and refresh before options

Some video= options might have a value that contains a dash. However, the
command line parsing mode considers all dashes as the separator between the
mode and the bpp count.

Let's rework the parsing code a bit to only consider a dash as the bpp
separator if it before a comma, the options separator.

A follow-up patch will add a unit-test for this once such an option is
introduced.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v4-12-60d38873f782@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>