drm/i915: spt does not have pch backlight override bit

SPT is always in the PCH override mode, and the bit MBZ. Only set
override on LPT.

v2: check for PCH version (Ville)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/i915/intel_panel.c
Simple merge