drm/i915: Introduce IS_GEN9_BC for Skylake and Kabylake.

Along with GLK it was introduced the .is_lp and IS_GEN9_LP.
So, following the same simplification standard we can
put Skylake and Kabylake under the same bucket for most
of the things.

So let's add the IS_GEN9_BC for "Big Core" (non Atom based
platforms).

The i915_drv.c was let out of this patch on purpose
because that is really a decision per platform, just like
other cases where IS_KABYLAKE is different from IS_SKYLAKE.

v2: fix conflict with IS_LP and 3 new cases for this
    big core bucket:
    - intel_ddi.c: intel_ddi_get_link_dpll
    - intel_fbc.c: find_compression_threshold
    - i915_gem_gtt.c: gtt_write_workarounds

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1485196357-30599-2-git-send-email-rodrigo.vivi@intel.com