drivers: gpu: Mark function as static in qxl_kms.c
Mark function qxl_device_init() as static in drm/qxl/qxl_kms.c because
it is not used outside this file.
This eliminates the following warning in drm/qxl/qxl_kms.c:
drivers/gpu/drm/qxl/qxl_kms.c:118:5: warning: no previous prototype for ‘qxl_device_init’ [-Wmissing-prototypes]