drm/vc4: hvs: Introduce a function to get the assigned FIFO

At boot time, if we detect that a pixelvalve has been enabled, we need to
be able to retrieve the HVS channel it has been assigned to so that we can
disable that channel too. Let's create that function that returns the FIFO
or an error from a given output.

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/178192d90874559b8386139f2226e773347729fc.1599120059.git-series.maxime@cerno.tech