drm/qxl: stop using TTM to call driver internal functions

The ttm_mem_io_* functions were intended to be internal to TTM and
shouldn't have been used in a driver. They were exported in commit
afe6804c just for QXL.

Instead call the qxl_ttm_io_mem_reserve() function directly and
completely drop the free call since that is a dummy on QXL.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/333289/