drm/i915/dp: fix compliance test adjustments

Abstract compliance test adjustments to a single function. Also make the
bpc adjustments affect the limits, actually forcing the bpc. Seems like
directly changing the pipe_bpp in the past could not have been
effective.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ef61e76003ab7719c82810b742f3fb5765c0e14c.1524730974.git.jani.nikula@intel.com