drm/qxl: remove set but not used variables 'driver, qbo'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/qxl/qxl_release.c: In function 'qxl_release_fence_buffer_objects':
drivers/gpu/drm/qxl/qxl_release.c:431:17: warning:
 variable 'qbo' set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/qxl/qxl_release.c:430:24: warning:
 variable 'driver' set but not used [-Wunused-but-set-variable]

'qbo' not used since commit f2c24b83 ("drm/ttm: flip the switch, and convert
to dma_fence")

And 'driver' never used since introduction in
8002db63

 ("qxl: convert qxl driver to proper use for reservations")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1542029556-88107-1-git-send-email-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>