drm: Remove obsolete GEM and PRIME callbacks from struct drm_driver

Several GEM and PRIME callbacks have been deprecated in favor of
per-instance GEM object functions. Remove the callbacks as they are
now unused. The only exception is .gem_prime_mmap, which is still
in use by several drivers.

What is also gone is gem_vm_ops in struct drm_driver. All drivers now
use struct drm_gem_object_funcs.vm_ops instead.

While at it, the patch also improves error handling around calls
to .free and .get_sg_table callbacks.

v3:
	* restore default call to drm_gem_prime_export() in
	  drm_gem_prime_handle_to_fd()
	* return -ENOSYS if get_sg_table is not set
	* drop all checks for obj->funcs
	* clean up TODO list and documentation
v2:
	* update related TODO item (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>