drm/pl111: Set .gem_prime_vmap and .gem_prime_mmap

These are needed for pl111 to use the generic fbdev emulation.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703160354.59955-4-noralf@tronnes.org