drm/vram-helper: Support struct drm_driver.gem_create_object

Drivers that what to allocate VRAM GEM objects with additional fields
can now do this by implementing struct drm_driver.gem_create_object.

v3:
	* separately check allocation failure in if/else branches
	  before upcast to gbo
v2:
	* only cast to gbo within if branch; set gbo directly
	  in else branch

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106125745.13797-9-tzimmermann@suse.de