drm/vc4: hdmi: Adjust HSM clock rate depending on pixel rate

The HSM clock needs to be setup at around 101% of the pixel rate. This
was done previously by setting the clock rate to 163.7MHz at probe time and
only check in mode_valid whether the mode pixel clock was under the pixel
clock +1% or not.

However, with 4k we need to change that frequency to a higher frequency
than 163.7MHz, and yet want to have the lowest clock as possible to have a
decent power saving.

Let's change that logic a bit by setting the clock rate of the HSM clock
to the pixel rate at encoder_enable time. This would work for the
BCM2711 that support 4k resolutions and has a clock that can provide it,
but we still have to take care of a 4k panel plugged on a BCM283x SoCs
that wouldn't be able to use those modes, so let's define the limit in
the variant.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7e692ddc231d33dd671e70ea04dd1dcf56c1ecb3.1599120059.git-series.maxime@cerno.tech