drm/i915/display: Group DC9 mask set

DC9 has a separate HW flow from the rest of the DC states and it is
available in GEN9 LP platforms and on GEN11 and newer, so here
moving the assignment of the mask to a single conditional block to
simplifly code.

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/20201111162408.98002-1-jose.souza@intel.com