drm/panfrost: Restructure the GEM object creation

Setting the GPU VA when creating the GEM object doesn't allow for any
conditional adjustments to the mapping. In preparation to support
adjusting the mapping and per FD address spaces, restructure the GEM
object creation to map and unmap the GEM object in the GEM object .open()
and .close() hooks.

While panfrost_gem_free_object() and panfrost_gem_prime_import_sg_table()
are not really needed after this commit, keep them as we'll need them in
subsequent commits.

Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808222200.13176-4-robh@kernel.org