drm/i915: remove check for aux irq

This became dead code with commit 309bd8ed ("drm/i915: Reinstate
GMBUS and AUX interrupts on gen4/g4x").

v2: Move comment about HW behavior to where decision is made to enable
MSI (Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180523180435.18042-1-lucas.demarchi@intel.com