drm/vkms: Add dumb operations

VKMS currently does not handle dumb data, and as a consequence, it does
not provide mechanisms for handling gem. This commit adds the necessary
support for gem object/handler and the dumb functions.

Changes since V1:
 Daniel Vetter:
 - Add dumb buffer support to the same patchset
Changes since V2:
 Haneen:
 - Add missing gem_free_object_unlocked callback to fix the warning
   "Memory manager not clean during takedown"

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/70b7becc91c6a323dbc15cb5fc912cbdfe4ef7d9.1531359228.git.rodrigosiqueiramelo@gmail.com