drm/i915/aux: use polling when irqs are unavailable

drm/i915/aux: use polling when irqs are unavailable

PTL with physically disconnected display was observed to have 40s longer
execution time when testing xe_fault_injection@xe_guc_mmio_send_recv.
The issue has not been seen when reverting commit 40a9f77a28fa ("Revert
"drm/i915/dp: change aux_ctl reg read to polling read"").

Apparently the configuration suffers from not having AUX enabled when
using interrupts. One probable cause can be xe enabling interrupts too
late: interrupts need memory allocations which currently can't be done
before the display FB takeover is done.

As for now, use polling for AUX in case interrupts are unavailable.

Fixes: 40a9f77a28fa ("Revert "drm/i915/dp: change aux_ctl reg read to polling read"")
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Michał Grzelak <michal.grzelak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260416163744.288107-1-michal.grzelak@intel.com
(cherry picked from commit 05e0550b65cd1604bd515fbc65f522bce4c10a87)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>