drm/vmwgfx: Fix prime import/export

vmwgfx never supported prime import of external buffers. Furthermore the
driver exposes two different objects to userspace: vmw_surface's and
gem buffers but prime import/export only worked with vmw_surfaces.

Because gem buffers are used through the dumb_buffer interface this meant
that the driver created buffers couldn't have been prime exported or
imported.

Fix prime import/export. Makes IGT's kms_prime pass.

Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Fixes: 8afa13a0

 ("drm/vmwgfx: Implement DRIVER_GEM")
Cc: <stable@vger.kernel.org> # v6.6+
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240412025511.78553-4-zack.rusin@broadcom.com