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>