drm/i915: Add support for pipe_bpp readout

On CTG+ read out the pipe bpp setting from hardware and fill it into
pipe config. Also check it appropriately.

v2: Don't do the pipe_bpp extraction inside the PCH only code block on
    ILK+.
    Avoid the PIPECONF read as we already have read it for the
    PIPECONF_EANBLE check.

Note: This is already in drm-intel-next-queued as
commit 42571aef
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Sep 6 23:29:00 2013 +0300

    drm/i915: Add support for pipe_bpp readout

but is needed for the following bugfix.

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