drm/hibmc: Update struct drm_driver for GEM object functions

The GEM object's free function is now called through struct
drm_gem_object_funcs.free. The function struct drm_driver.gem_prime_mmap
is now required for mmap'ing GEM objects to userspace.

v2:
	* set drm_driver.gem_prime_mmap to drm_gem_prime_mmap()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190702115012.4418-4-tzimmermann@suse.de