drm/amd/display: Don't attempt to program missing register fields on DCE8
When moving to a common dce/ infrastructure for all asics, some register fields
do not exist in DCE8, and cause ASSERTS and debug spam.
Instead, check to see whether a register field mask is valid before attempting
to program the register field
Signed-off-by: Jordan Lazare <Jordan.Lazare@amd.com> Reviewed-by: Tony Cheng <Tony.Cheng@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>