drm/i915/fbc: add comments to the FBC auxiliary structs

I wrote this code an year and a half ago and I couldn't exactly
remember the main differences of these two structures when reviewing a
new FBC patch. Add some comments to help explain what's the purpose of
each struct.

For the record, the original commits are:
 b183b3f1 ("drm/i915/fbc: introduce struct intel_fbc_reg_params")
 aaf78d27

 ("drm/i915/fbc: introduce struct intel_fbc_state_cache")

Cc: Praveen Paneri <praveen.paneri@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170714193822.12121-1-paulo.r.zanoni@intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>