drm/amd/display: Limit DCN to x86 arch

DCN bw calcs currently rely on the following gcc options:
  -mhard-float -msse -mpreferred-stack-boundary=4

We probably shouldn't really try building this on architectures
other than x86.

CC: Alex Deucher <Alexander.Deucher@amd.com>
CC: Christian König <christian.koenig@amd.com>
CC: Michel Dänzer <michel.daenzer@amd.com>
CC: Tony Cheng <Tony.Cheng@amd.com>
CC: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>