drm/i915/fbc: refactor some small functions called only once

The FBC fixes we've been doing in the last months required a lot of
refactor, so functions that were once big and called from different
spots are now small and called only once. IMHO now it's better to just
move the contents of these functions to their only callers since this
reduces the number of indirections while reading the code.

While at it, also improve the related comments a little bit.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-26-git-send-email-paulo.r.zanoni@intel.com