drm/i915: change no_fbc_reason from enum to string

drm/i915: change no_fbc_reason from enum to string

I wanted to add yet another check to intel_fbc_update() and realized
I would need to create yet another enum no_fbc_reason case. So I
remembered this patch series that Damien wrote a long time ago and
nobody ever reviewed, so I decided to reimplement it since the code
changed a lot since then.

Credits-to: Damien Lespiau <damien.lespiau@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445964628-30226-2-git-send-email-paulo.r.zanoni@intel.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>