drm/scheduler: make unexported items static

The drm_sched_fence_ops_{scheduled,finished} are not exported
from the file so make them static to avoid the following
warnings from sparse:

drivers/gpu/drm/scheduler/sched_fence.c:131:28: warning: symbol 'drm_sched_fence_ops_scheduled' was not declared. Should it be static?
drivers/gpu/drm/scheduler/sched_fence.c:137:28: warning: symbol 'drm_sched_fence_ops_finished' was not declared. Should it be static?

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191009121447.31017-1-ben.dooks@codethink.co.uk