drm/i915: re-order if/else ladder for hpd_irq_setup

Use the convention of new platforms first. No need to special case
HAS_GMCH() since that stopped being true at the lattest on gen8 (for
cherryview).

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106225531.920641-6-lucas.demarchi@intel.com