drm/i915: Extract intel_hpd_enabled_irqs()

Eliminate a bunch of duplicated code that calculates the currently
enabled HPD interrupt bits.

v2: s/;/:/ in patch subject (Paulo)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>