drm/mgag200: Align register field names with documentation

In mgag200_set_mode_regs(), align variable names with the field names
given in the Matrox programming manuals. Makes the code and docs grep-
able.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718104551.575912-3-tzimmermann@suse.de