drm/sun4i: tcon: Add TRI finish interrupt for vblank

The "CPU" (or Intel 8080) interface uses a different interrupt called
TRI_FINISH (most likely TRI being for trigger) to notify the end of frames,
and hence the VBLANK period.

And that interrupt to the possible VBLANK interrupts source.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/de6d6ad8959da77ea3a974a31a4c0c8391178748.1522835818.git-series.maxime.ripard@bootlin.com