drm/amd/display: enable low power mode for VGA memory

[Why]
When unused, all memory blocks should be put in a low power state

[How]
Check the value of enable_mem_low_power.bits.vga and set
corresponding bit in the hardware register
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn30/dcn30_hwseq.c

@@ -461,6 +461,11 @@ void dcn30_init_hw(struct dc *dc)
 		REG_SET_2(ODM_MEM_PWR_CTRL3, 0, ODM_MEM_UNASSIGNED_PWR_MODE, 3, ODM_MEM_VBLANK_PWR_MODE, 1);
 	}
 
+	if (dc->debug.enable_mem_low_power.bits.vga) {
+		// Power down VGA memory
+		REG_UPDATE(MMHUBBUB_MEM_PWR_CNTL, VGA_MEM_PWR_FORCE, 1);
+	}
+
 	if (dc->ctx->dc_bios->fw_info_valid) {
 		res_pool->ref_clocks.xtalin_clock_inKhz =
 				dc->ctx->dc_bios->fw_info.pll_info.crystal_frequency;