drm/i915: Handle HPD when it has actually occurred

Writing to PCH_PORT_HOTPLUG for each interrupt is not required.
Handle it only if hpd has actually occurred like we handle other
interrupts.
v2: Make few variables local to if block (Ville)
v3: Add check for ibx/cpt both (Ville).
    While at it, remove the redundant check for hotplug_trigger from
    pch_get_hpd_pins
v4: Indentation (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>