drm/msm/dpu: rework documentation comments

Unfortunately the tooling doesn't check documents placed before funciton
prototypes. Such comments frequently become outdated, miss several
params, etc. Move documentation for the functions to be placed before
the actual function body, allowing 'make W=1' to actually check these
comments and report an error.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/622690/
Link: https://lore.kernel.org/r/20241102-dpu-docs-rework-v1-1-d735853fd6db@linaro.org