drm/udl: introduce a macro to convert dev to udl.

commit fd96e0dba19c53c2d66f2a398716bb74df8ca85e upstream.

This just makes it easier to later embed drm into udl.

[rez] Regarding the backport to v4.14.y, the only difference is due to
the fact that in v4.14.y the udl_gem_mmap() function doesn't have a
local 'struct udl_device' pointer so it didn't need to be converted.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405031715.5959-3-airlied@gmail.com


Signed-off-by: Ross Zwisler <zwisler@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>