The mgag200 driver maps registers into the address space. Move the
code into a separate helper function. No functional changes.

One small difference is in the handling of SDRAM/SGRAM. MGA devices
can come with either SDRAM or SGRAM. So far, the driver checked for
SDRAM, which is the common case. The patch moves this code into a
separate helper and checks for SGRAM, which is the special case. The
test itself is the same as before.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730102844.10995-3-tzimmermann@suse.de