drm/amd/powerplay: Add PP_CAP() macro

To replace common lengthy sequence that would create
really long lines all over.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>