Drivers must fill out the handle_to_fd and fd_to_handle hooks to
enable export/import prime functionality already. The additional
DRIVER_PRIME flag doesn't serve any real purpose, since the overall
flag doesn't even tell you whether import or export or maybe even both
is supported.

Ditch it.

This patch just makes it defunct, subsequent patches will remove it
from all the drivers.

Note this changes the userspace visible errno in some situations from
EOPNOTSUPP to ENOSYS. Userspace already needs to treat both as "no
prime support", so this should break anything.

v2: Improve commit message as suggested by Emil
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-6-daniel.vetter@ffwll.ch