drivers: display: stm32_ltdc: mask irq if necessary

LTDC interrupt routine is used to reload frame buffer pointer
once full frame is finished flushing. As long as there is no
need to change buffer - there is no need to disturb CPU.
Thus: Enable LTDC interrupt only when new buffer is pending

Signed-off-by: Pavlo Hamov <pasha.gamov@gmail.com>