drm/bridge: sii902x: Refactor init code into separate function

Separate the hardware initialization code from setting up the data
structures and parsing the device tree. The purpose of this change is
to provide a single exit point and avoid a waterfall of 'goto's in
the subsequent patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201020221501.260025-1-mr.nuke.me@gmail.com