drm/mgag200: Add dedicated variables for blanking fields

Represent fields for horizontal and vertical blanking with <hblkstr>,
<hblkend>, <vblkstr> and <vblkend>. Aligns the code with the Matrox
programming manuals.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718104551.575912-5-tzimmermann@suse.de