drm/amd/pm: Add custom/current freq to pstates

Add custom member for user requested custom frequency, level mask
or min/max frequencies. Add curr member to keep track of the current
active values.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>