drm/v3d: Use the new shmem helpers to reduce driver boilerplate.

The new shmem helpers from Noralf and Rob abstract out a bunch of our
BO creation and mapping code.

v2: Use the new sgt getter, and flag pages as dirty before freeing.
v3: Remove the mismatched put_pages.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190314163451.13431-1-eric@anholt.net
Reviewed-by: Rob Herring <robh@kernel.org> (v2)