drm/dumb-buffers: Add defaults for .dumb_map_offset and .dumb_destroy

Almost everyone did end up using GEM as bo, so this adds defaults
for the drm_driver.dumb_destroy and drm_driver.dumb_map_offset
callbacks.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500837417-40580-3-git-send-email-noralf@tronnes.org