drm/i915: don't try to find crtcs for FBC if it's disabled

.. because it would be a waste of time, so move the place where the
check is done. Also, with this we won't risk printing "more than one
pipe active, disabling compression" or "no output, disabling" when FBC
is actually disabled.

This patch also represents a small behavior difference when using
i915.powersave=0: it is now exactly the same as i915.enable_fbc=0 on
this part of the code.

V2: Rebase.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> (v1)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>