drivers: gpu: drm: i915: intel_fifo_underrun.c: Fix a typo in comment

The comment for intel_cpu_fifo_underrun_irq_handler() is not consistent
with the code and the rest of the comment for this routine. This patch
fixes this typo in comment.

Signed-off-by: Kumar Amit Mehta <gmate.amit@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>