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>

drivers/gpu/drm/amd/display/dc/dml2/dml21/src/dml2_core/dml2_core_dcn4_calcs.c

@@ -6887,7 +6887,8 @@ struct dml2_core_internal_g6_temp_read_blackouts_table {
 	} entries[DML_MAX_CLK_TABLE_SIZE];
 };
 
-struct dml2_core_internal_g6_temp_read_blackouts_table core_dcn4_g6_temp_read_blackout_table = {
+static const struct dml2_core_internal_g6_temp_read_blackouts_table
+	core_dcn4_g6_temp_read_blackout_table = {
 	.entries = {
 		{
 			.uclk_khz = 96000,