drm/i915: Add port A HPD support for BDW

Wire up the port A HPD for BDW. Compared to earlier platforms the
interrupt setup is a bit different, but basically everything else
looks the same.

v2: 0 initialize pin_mask/long_mask due to intel_get_hpd_pins() changes
    Check for BDW before processing the HPD to not break BXT
    Set found=true when processing port A HPD
    Sort out the mess I made of the irq setup in v1
    Warn about bad irq mask vs. enable bits in bdw_update_port_irq()  (Paulo)

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