drm/i915/icl: avoid unclaimed PLANE_NV12_BUF_CFG register

We don't have proper watermark NV12 support on ICL due to differences
in how it should be implemented. In commit 234059da
("drm/i915/icl: NV12 y-plane ddb is not in same plane") we avoided
writing the non-existent PLANE_NV12_BUF_CFG registers but we forgot to
also avoid them on the hardware state readout. While the code is still
not correct, at least now we can avoid unclaimed register error
messages when dealing with RGB formats, which makes CI happier.

Also add some FIXME comments in order to make it even more clear that
there's still work to do.

References: commit 234059da ("drm/i915/icl: NV12 y-plane ddb is
 not in same plane")
Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180801004614.22149-1-paulo.r.zanoni@intel.com