matroxfb: restore the registers M_ACCESS and M_PITCH

commit a772d4736641ec1b421ad965e13457c17379fc86 upstream.

When X11 is running and the user switches back to console, the card
modifies the content of registers M_MACCESS and M_PITCH in periodic
intervals.

This patch fixes it by restoring the content of these registers before
issuing any accelerator command.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>