Loop N1 instruction delay for burst mode devices are computed
based on horizontal sync and porch timing values.

The current driver is using u16 type for computing this hsync_porch
value, which would failed to fit within the u16 type for large sync
and porch timings devices. This would result in hsync_porch overflow
and eventually computed wrong instruction delay value.

Example, timings, where it produces the overflow
{
	.hdisplay       = 1080,
	.hsync_start    = 1080 + 408,
        .hsync_end      = 1080 + 408 + 4,
        .htotal         = 1080 + 408 + 4 + 38,
}

It reproduces the desired delay value 65487 but the correct working
value should be 7.

So, Fix it by computing hsync_porch value separately with u32 type.

Fixes: 1c1a7aa3 ("drm/sun4i: dsi: Add burst support")
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190512184128.13720-2-jagan@amarulasolutions.com