drm/udl: Use GEM vmap/mmap function from SHMEM helpers

The udl driver contains an implementation of GEM vmap and mmap
operations that is identical to the common SHMEM helper; except
that udl's code uses cached pages by default.

Convert udl to regular SHMEM helper functions. There's no reason
to have udl behave differently from all other SHMEM drivers. The
udl driver uses the SHMEM helper to enable caching.

v3:
	* rebased onto Daniel's shmem untangle series
v2:
	* implement .gem_create_object with SHMEM helper

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200609090820.20256-3-tzimmermann@suse.de