drm/i915/display: Make get_allowed_dc_mask().max_dc set a chain of if and elses

Just following what we do in many other places, DG1 is a exception so
move it to the top instead of add it inside of INTEL_GEN(dev_priv) >= 12.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201111162408.98002-2-jose.souza@intel.com