drm/mgag200: Protect concurrent access to I/O registers with lock

Add a mutex lock to protect concurrent access to I/O registers
against each other. This happens between invocation of commit-
tail functions and get-mode operations. Both with use the CRTC
index registers MGA1064_GEN_IO_DATA and MGA1064_GEN_IO_CTL.
Concurrent access can lead to failed mode-setting operations.

v2:
	* fix typo in commit description (Jocelyn)
	* add comment to explain rmmio_lock

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220502142514.2174-4-tzimmermann@suse.de