drm: exynos: dsi: Add atomic check

Look like an explicit fixing up of mode_flags is required for DSIM IP
present in i.MX8M Mini/Nano SoCs.

At least the LCDIF + DSIM needs active low sync polarities in order
to correlate the correct sync flags of the surrounding components in
the chain to make sure the whole pipeline can work properly.

On the other hand the i.MX 8M Mini Applications Processor Reference Manual,
Rev. 3, 11/2020 says.
"13.6.3.5.2 RGB interface
 Vsync, Hsync, and VDEN are active high signals."

i.MX 8M Mini Applications Processor Reference Manual Rev. 3, 11/2020
3.6.3.5.2 RGB interface
i.MX 8M Nano Applications Processor Reference Manual Rev. 2, 07/2022
13.6.2.7.2 RGB interface
both claim "Vsync, Hsync, and VDEN are active high signals.", the
LCDIF must generate inverted HS/VS/DE signals, i.e. active LOW.

No clear evidence about whether it can be documentation issues or
something, so added proper comments on the code.

Comments are suggested by Marek Vasut.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>