amdgpu/dc: move filter taps to being static const data (v2)

This just adds two accessor methods, and moves all the data
to static const.

v2: fix dcn build.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>