drm/vc4: hdmi: Remove redundant variables

The vc4_hdmi_audio_prepare function and the functions it's calling have
in several occurences multiple dereferences of either the sample rate or
the number of channels.

It turns out that these variables are also passed through the hdmi codec
parameters structure. Convert all the users to use this structure, and
if it's used multiple times use a variable to store it instead of
dereferencing it every time.

Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210707093632.1468127-1-maxime@cerno.tech