drm/i915: Add wrappers to handle fifo underrun interrupts

Way too much copypasta all over. And this also clarifies a bit what's
going on since it separates the "do we have an underrun irq" from the
"should we report the underrun" check.

v2: Fix excessively long lines.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>