drm: Add VRAM MM, a simple memory manager for dedicated VRAM

drm: Add VRAM MM, a simple memory manager for dedicated VRAM

The VRAM MM memory manager is a helper library that manages dedicated video
memory of simple framebuffer devices. It is supported to be used with
struct drm_gem_vram_object, but does not depend on it.

The implementation is based on the respective code from ast, bochs, and
mgag200. These drivers share the exact same implementation except for type
names. The helpers are currently build with TTM. This may change in future
revisions.

v4:
	* cleanups from checkpatch.pl
v2:
	* renamed to struct drm_vram_mm
	* add drm_vram_mm_mmap() helper
	* documentation fixes

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-7-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>