drm/vc4: hdmi: Add pixel BVB clock control

The BCM2711 has another clock that needs to be ramped up depending on the
pixel rate: the pixel BVB clock. Add the code to adjust that clock when
changing the mode.

[Maxime: Changed the commit log, used clk_set_min_rate]

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20200901040759.29992-3-hoegeun.kwon@samsung.com
Link: https://patchwork.freedesktop.org/patch/msgid/d757ddd6549da140f178563e5fd2bf1d129913fd.1599120059.git-series.maxime@cerno.tech