drm/i915: Refactor .hpd_irq_setup() calls a bit

Add a small wrapper for .hpd_irq_setup() which does the
"do we even have the hook?" and "are display interrupts enabled?"
checks.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006185809.4655-3-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>