drm/i915: move dsi init functions to intel_dsi.h

Keep the header self-contained. It'll need rework of its own in the
future, but gotta start somewhere.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/158347ef715a35ca1f7d945efb139d80bf5e0e6c.1556540889.git.jani.nikula@intel.com