drm/i915: Don't call intel_get_hpd_pins() when there's no hotplug interrupt

drm/i915: Don't call intel_get_hpd_pins() when there's no hotplug interrupt

On GMCH plaforms we are now getting the following spew on aux
interrupts:
[drm:intel_get_hpd_pins] hotplug event received, stat 0x00000000, dig 0x00000000, pins 0x00000000
[drm:intel_get_hpd_pins] hotplug event received, stat 0x00000000, dig 0x00000000, pins 0x00000000
[drm:intel_get_hpd_pins] hotplug event received, stat 0x00000000, dig 0x00000000, pins 0x00000000
[drm:intel_get_hpd_pins] hotplug event received, stat 0x00000000, dig 0x00000000, pins 0x00000000
[drm:intel_get_hpd_pins] hotplug event received, stat 0x00000000, dig 0x00000000, pins 0x00000000
[drm:intel_dp_aux_ch] dp_aux_ch timeout status 0x71450064

Prevent it by not calling intel_get_hpd_pins() unless one of the HPD
interrupt bits are actually set.

I already fixed similar annoyance once with
61d3c50af6efc508a1c6177bac4f3540108d3085 drm/i915: Use HOTPLUG_INT_STATUS_G4X on VLV/CHV

but another source for it got added in
380a2c88dee31363fa8dde3ad8697445b02131d0 drm/i915: combine i9xx_get_hpd_pins and pch_get_hpd_pins

due to pch_get_hpd_pins() being chosen over i9xx_get_hpd_pins() to
serve as the new unified piece of code. pch_get_hpd_pins() had the debug
print, and i9xx_get_hpd_pins() didn't.

Cc: Imre Deak <imre.deak@intel.com>
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>