drm/i915: Disable CPU underruns around eDP port and vdd enable on ILK-IVB

We sometimes get a spurious CPU pipe underrun somewhere between
enabling port A and enabling vdd for the panel. Observed on both
ILK and IVB with port A eDP. Suppress FIFO underrun reporting
around the port and vdd enable to avoid the dmesg errors.

Not sure if port D eDP would suffer from the same issue, but assume
that it doesn't until proven differently.

Testcase: igt/kms_setmode
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448050160-14124-2-git-send-email-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>