drm/xe: Disallow pinning dma-bufs in VRAM

drm/xe: Disallow pinning dma-bufs in VRAM

For now only support pinning in TT memory, for two reasons:
1) Avoid pinning in a placement not accessible to some importers.
2) Pinning in VRAM requires PIN accounting which is a to-do.

v2:
- Adjust the dma-buf kunit test accordingly.

Suggested-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920095001.5539-1-thomas.hellstrom@linux.intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>