drm/amd/display: Use channel_width = 2 for vram table 3.0

VBIOS has suggested to use channel_width=2 for any ASIC that uses vram
info 3.0. This is because channel_width in the vram table no longer
represents the memory width

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>