drm/mgag200: Use hexadecimal register indeces

In mgag200_set_mode_regs(), use hexadecimal indices for accessing
registers. Aligns the code with the register indices in the Matrox
programming manuals. Also convert to lower-case hexadecimal values.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718104551.575912-2-tzimmermann@suse.de