drm/i915/skl: drop workarounds for A0 and B0 revisions

Pre-production hardware is not supported.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/7929af62a68504c84038a8db1625bd96ebaa9e6f.1474034059.git.jani.nikula@intel.com