drm/i915/dsi: be defensive about out of bounds operation byte

Untie the VBT based generic panel driver from the VBT parsing, so that
the two don't have to be updated in lockstep.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/01c71ac89a9db8bc7b8ae0fb05c50a5fae362dc4.1450702954.git.jani.nikula@intel.com