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>