drm/mediatek: Introduce GEM object functions

GEM object functions deprecate several similar callback interfaces in
struct drm_driver. This patch replaces the per-driver callbacks with
per-instance callbacks in mediatek. The only exception is gem_prime_mmap,
which is non-trivial to convert.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>