drm/i915: Introduce FBC DocBook.

No functional changes.

v2 (Paulo): Rebase.
v3: Accept Daniel's suggestions:
    * remove unclear and duplicated explanation.
    * remove marketing like doc and replace by a simple one.
    * remove bdw_fbc_sw_flush documentation.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>