drm/vc4: crtc: Add function to compute FIFO level bits

The longer FIFOs in vc5 pixelvalves means that the FIFO full level
doesn't fit in the original register field and that we also have a
secondary field. In order to prepare for this, let's move the registers
fill part to a helper function.

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/e46a3823128af50c1c833de8fa9b95e9b86c2f66.1599120059.git-series.maxime@cerno.tech