drm/i915/display: convert intel_fifo_underrun.[ch] to struct intel_display

Going forward, struct intel_display is the main display device data
pointer. Convert as much as possible of intel_fifo_underrun.[ch] to
struct intel_display.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/682e667013e1726a6f2f78484b7e9618cee3b639.1739378096.git.jani.nikula@intel.com