drm/amd/display: fix regamma build optimization

[Why]
When the global variable pow_buffer_ptr is set to -1, by definition
optimizations should not be used to build the regamma. Since
translate_from_linear_space unconditionally increments this global, it
inadvertently enables the optimization.

[How]
Increment pow_buffer_ptr only if it is not -1.
Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/modules/color/color_gamma.c

@@ -364,8 +364,10 @@ static struct fixed31_32 translate_from_linear_space(
 			scratch_2 = dc_fixpt_mul(gamma_of_2,
 					pow_buffer[pow_buffer_ptr%16]);
 
-		pow_buffer[pow_buffer_ptr%16] = scratch_2;
-		pow_buffer_ptr++;
+		if (pow_buffer_ptr != -1) {
+			pow_buffer[pow_buffer_ptr%16] = scratch_2;
+			pow_buffer_ptr++;
+		}
 
 		scratch_1 = dc_fixpt_mul(scratch_1, scratch_2);
 		scratch_1 = dc_fixpt_sub(scratch_1, args->a2);