drm/sun4i: rgb: Change the pixel clock validation check

drm/sun4i: rgb: Change the pixel clock validation check

The current code, since commit bb43d40d7c83 ("drm/sun4i: rgb: Validate the
clock rate"), perform some validation on the pixel clock to filter out the
EDID modes provided by monitors (through bridges) that we wouldn't be able
to reach. For the usual modes, we're able to generate a perfect clock rate,
so a strict check was enough.

However, this had the side effect of preventing displays that would work
otherwise to operate properly, since we would pretty much never be able to
generate an exact rate for those displays, even though we would fall within
that panel tolerance.

This was also shown to happen for unusual modes exposed through EDIDs, for
example on eDP panels.

We can work around this by simplifying a bit the problem: no panels we've
encountered so far actually needed that check. All of them are tied to a
particular board when it is produced, and made to work with the Allwinner
BSP. That pretty much guarantees that we never have a pixel clock out of
reach.

On the other hand, the EDIDs modes that needed to be validated have always
been exposed through bridges.

Let's just use that metric to instead of validating all modes, only
validate modes when we have a bridge attached. It should be good enough for
now, while we still have room for improvements or refinements using the
display_timings structure for example for panels.

We also add a tolerance for EDID-based modes instead of doing a strict
check. This tolerance is of 0.5% which is the one advertised in the VESA
DVT and CVT specs. If that needed to be extended in the future, we can add
a custom module parameter to relax it a bit.

Fixes: bb43d40d7c83 ("drm/sun4i: rgb: Validate the clock rate")
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Vasily Khoruzhick <anarsoul@gmail.com> # tested on pinebook
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ec2dc2a7b3d4bd44f7a2a6e1c1813f92449a7310.1551191081.git-series.maxime.ripard@bootlin.com