drm/i915: unify no_fbc_reason message printing

This commit has two main advantages: simplify intel_fbc_update()
and deduplicate the strings.

v2:
 - Rebase due to changes on P1.
 - set_no_fbc_reason() can now return void (Chris).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>