drm/i915: extract crtc_is_valid() on the FBC code

We're going to kill intel_fbc_find_crtc(), that's why a big part of
the logic moved from intel_fbc_find_crtc() to crtc_is_valid().

v2:
  - Rebase due to pipe_a_only change.
  - Split the multiline conditional (Chris).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446664257-32012-5-git-send-email-paulo.r.zanoni@intel.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>