amdgpu/dc: use the builtin constant p trick on the 31/32 fixed point.

This only gets us 100 bytes, but may as well be consistent.

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>