drm: qxl: Remove unused device pointer

Remove occurrences of unused struct qxl_device pointer in functions
qxl_ttm_fault() and qxl_init_mem_type().

Detected by Coverity: CID 1019128, CID 1019129.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Dave Airlie <airlied@redhat.com>