drm/mgag200: Simplify offset calculation

drm/mgag200: Simplify offset calculation

The offset value sets the distance in bytes between two consecutive
scanlines. Reduce the calculation to the minimum.

According to the Matrox programming manual, Sec 4.6.5, the offset
is the scanline pitch in bits divided by 128. The field pitches[0] in
struct drm_framebuffer stores the scanline pitch in bytes, so we have
to divide by 16 only. Reducing the existing bpp-shift look-up and
offset calculations also returns exactly this for all formats.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Co-developed-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260415152625.101710-4-tzimmermann@suse.de