drm: zynqmp_dp: Split off several helper functions

In preparation for supporting compliance testing, split off several
helper functions. No functional change intended.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809193600.3360015-7-sean.anderson@linux.dev