drm/i915: extract AUX mask assignment to separate function

No change in behavior, this only allows to more easily follow the flow
of gen8_de_irq_handler without the mask assignments for each platform.
This also re-organizes the branches a little bit, so the one-off case
for CNL_WITH_PORT_F is separate from the generic gen >= 11.

v2: rename de_port_iir_aux_mask -> gen8_de_port_aux_mask (Ville)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jose Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190226004900.26047-1-lucas.demarchi@intel.com