drm/i915: Make intel_crtc_arm_fifo_underrun() functional on gen2

Assuming intel_crtc_arm_fifo_underrun() only gets called when
there's no pending plane updates we can utilize it on gen2 by
checking the active_planes bitmask so that we only re-enable
underrun reporting if some planes are active.
i915_fifo_underrun_reset_write() seems to have the necessary
hw_done/flip_done waits in place.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127190556.1574-8-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>