drm/i915: read bpp from vbt only for older panels

BPP bits defined in VBT should be used only on panels whose
edid version is 1.3 or older. EDID version 1.4 introduced offsets
where bpp is defined and read into display_info, hence bpp from
VBT will be used only when bpc in display_info is zero.

v2: use display_info.bpc for deciding when to use vbt_bpp (Jani)

Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>