drm/mgag200: Use adjusted mode values for CRTCs

Use the values with the crtc_ prefix from struct drm_display_mode to
program hardware. The DRM core adjusted these values to the requirements
of CRTC hardware.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240718104551.575912-4-tzimmermann@suse.de