fbdev: sh_mipi_dsi: add VMLEN1/VMLEN2 calculation

VMLEN1/VMLEN2 needs blanking length which is calculated from
video image size.

The calculation methods are explained on
[SH MIPI] - [Video Mode] - [Blanking Packet setting]
But HFPLEN (= VMLEN2) is un-understandable.

For example, if SH-MIPI input was RGB888 (3byte),
output was RGB888 (3byte)
and 3lane connection, the date goes straight.

But if SH-MIPI input was RGB888 (3byte),
output was RGB565 (2byte)
and 4lane connection, it needs delay in HFPLEN.
Then (input cycle - output cycle) * lane is necessary the delay

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>