drm/mgag200: Acquire I/O lock while reading EDID

DDC operation conflicts with concurrent mode setting. Acquire the
driver's I/O lock in get_modes to prevent this. This change should
have been part of commit 931e3f3a ("drm/mgag200: Protect
concurrent access to I/O registers with lock"), but apparently got
lost somewhere.

v3:
	* fix commit message to say 'drm/mgag200' (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 931e3f3a ("drm/mgag200: Protect concurrent access to I/O registers with lock")
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220516134343.6085-2-tzimmermann@suse.de