drm/ttm: use GEM references for VM mappings

Instead of a TTM reference grab a GEM reference whenever necessary for a
VM mapping.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240723121750.2086-7-christian.koenig@amd.com