drm/amd/display: fix gamma not being applied correctly

[why]
Gamma was always being set as identity on SDR monitor,
leading to no changes in gamma. This caused nightlight to
not apply correctly.

[how]
Added a default gamma structure to compare against
in the sdr case.

Signed-off-by: Murton Liu <murton.liu@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c

@@ -1227,7 +1227,8 @@ static bool dcn10_set_input_transfer_func(struct pipe_ctx *pipe_ctx,
 		tf = plane_state->in_transfer_func;
 
 	if (plane_state->gamma_correction &&
-		!plane_state->gamma_correction->is_identity
+		!dpp_base->ctx->dc->debug.always_use_regamma
+		&& !plane_state->gamma_correction->is_identity
 			&& dce_use_lut(plane_state->format))
 		dpp_base->funcs->dpp_program_input_lut(dpp_base, plane_state->gamma_correction);