drm/vgem: Enable dmabuf interface for export

Enable the standard GEM dma-buf interface provided by the DRM core, but
only for exporting the VGEM object. This allows passing around the VGEM
objects created from the dumb interface and using them as sources
elsewhere. Creating a VGEM object for a foriegn handle is not supported.

v2: With additional completeness.
v3: Need to clear the CPU cache upon exporting the dma-addresses.
v4: Use drm_gem_put_pages() as well.
v5: Use drm_prime_pages_to_sg()

Testcase: igt/vgem_basic/dmabuf-*
Testcase: igt/prime_vgem
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Zach Reizner <zachr@google.com>
Acked-by: Zach Reizner <zachr@google.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468242488-1505-3-git-send-email-chris@chris-wilson.co.uk