fbmon: use VESA_DMT_VSYNC_HIGH to fix typo

VESA_DMT_VSYNC_HIGH should be used instead of VESA_DMT_HSYNC_HIGH,
because FB_SYNC_VERT_HIGH_ACT is related to vsync, not to hsync.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>