drm/i915: Skip PIPESTAT reads from irq handler on VLV/CHV when power well is down

PIPESTAT registers live in the display power well on VLV/CHV, so we
shouldn't access them when things are powered down. Let's check
whether the display interrupts are on or off before accessing the
PIPESTAT registers.

Another option would be to read the PIPESTAT registers only when
the IIR register indicates that there's a pending pipe event. But
that would mean we might miss even more underrun reports than we
do now, because the underrun status bit lives in PIPESTAT but doesn't
actually generate an interrupt.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93738
Cc: Chris Wilson <[email protected]>
Tested-by: Chris Wilson <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Imre Deak <[email protected]>

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