drm: Add drm_gem_vram_fill_create_dumb() to create dumb buffers

drm: Add drm_gem_vram_fill_create_dumb() to create dumb buffers

The helper function drm_gem_vram_fill_create_dumb() implements most of
struct drm_driver.dumb_create() for GEM-VRAM buffer objects. It's not a
full implementation of the callback, as several driver-specific parameters
are still required.

v4:
	* cleanups from checkpatch.pl
v2:
	* documentation fixes

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190508082630.15116-5-tzimmermann@suse.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>