drm/i915: reduce includes in intel_fifo_underrun.h

Only include what's needed.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/29b57e168e8af842baad2626959cea258402a2c1.1667383630.git.jani.nikula@intel.com