drm/amd/display: Use macro for invalid OPP ID

[WHY]
This is meant to make it clearer that 0xf is not a valid OPP ID, and
that code making use of OPP IDs should not accept this value.

Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>