drm/modes: parse_cmdline: Add support for named modes containing dashes

drm/modes: parse_cmdline: Add support for named modes containing dashes

It is fairly common for named video modes to contain dashes (e.g.
"tt-mid" on Atari, "dblntsc-ff" on Amiga).  Currently such mode names
are not recognized, as the dash is considered to be a separator between
mode name and bpp.

Fix this by skipping any dashes that are not followed immediately by a
digit when looking for the separator.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v4-13-60d38873f782@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>