drm: Add simple PRIME helpers for GEM VRAM

These basic helper functions for GEM VRAM allow for pinning and mapping
GEM VRAM objects via the PRIME interfaces. It's not a full implementation,
but complete enough for generic fbcon.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-6-tzimmermann@suse.de


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