drm/amd/display: make two symbols static

This addresses the following sparse warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_hw_sequencer.c:2740:6:
warning: symbol 'dce110_set_cursor_position' was not declared. Should it
be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dce110/dce110_hw_sequencer.c:2785:6:
warning: symbol 'dce110_set_cursor_attribute' was not declared. Should
it be static?

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>