drm: hdlcd: Fix the picture tear when flipping framebuffers.

HDLCD contains logic in hardware to wait for VSYNC before updating
the start of framebuffer register, so we don't need to do it in
software. More than that, to remove any tearing we need to wait
for VSYNC after setting up a new framebuffer as releasing the old
fb while the hardware is still using it can cause artifacts.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>