UPSTREAM: drm/prime: Add drm_gem_prime_mmap()

Add a generic PRIME GEM mmap function.

v2: Fix link in docs (Daniel Vetter)

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181110145647.17580-3-noralf@tronnes.org


(cherry picked from commit 7698799f9554deeb0f3abb22857c3deb41c10db8)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I0322d5051ce4db5ac3ded3d367c0f261864b1bb1