drm/mgag200: Replace mapping code with drm_gem_vram_{kmap/kunmap}()

The mgag200 driver establishes several memory mappings for frame buffers
and cursors. This patch converts the driver to use the equivalent
drm_gem_vram_kmap() functions. It removes the dependencies on TTM
and cleans up the code.

v4:
	* cleanups from checkpatch.pl

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-17-tzimmermann@suse.de


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>