drm/amd/display: Make core_dcn4_g6_temp_read_blackout_table static

drm/amd/display: Make core_dcn4_g6_temp_read_blackout_table static

The sparse tool complains as follows:

drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/dml21/src/dml2_core/dml2_core_dcn4_calcs.c:6853:56: warning:
	symbol 'core_dcn4_g6_temp_read_blackout_table' was not declared. Should it be static?

This symbol is not used outside of dml2_core_dcn4_calcs.c, so marks it static.

And not want to change it, so mark it const.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>