drm/amd/display: assert if mask is 0 in set_reg_field_value_ex

-  mask = 0 means something is wrong in caller and no register field will be updated

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>