drm/vc4: crtc: Deal with different number of pixel per clock

Some of the HDMI pixelvalves in vc5 output two pixels per clock cycle.
Let's put the number of pixel output per clock cycle in the CRTC data and
update the various calculations to reflect that.

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: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/18a3bb079981ba820132b37e736a4bb371234d2e.1599120059.git-series.maxime@cerno.tech