drm/i915: Move common PCH_PP_CONTROL setup to ironlake_get_pp_control

Every usage of PCH_PP_CONTROL sets the PANEL_UNLOCK_REGS value to
ensure that writes will be respected, move this to a common function
to make the driver cleaner.

No functional changes.

Signed-off-by: Keith Packard <keithp@keithp.com>