drm/vmwgfx: Add basic support for external buffers

Make vmwgfx go through the dma-buf interface to map/unmap imported
buffers. The driver used to try to directly manipulate external
buffers, assuming that everything that was coming to it had to live
in cpu accessible memory. While technically true because what's in the
vms is controlled by us, it's semantically completely broken.

Fix importing of external buffers by forwarding all memory access
requests to the importer.

Tested by the vmw_prime basic_vgem test.

Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722184313.181318-5-zack.rusin@broadcom.com