drm/i915/fbc: extract intel_fbc_can_enable()

Make our enable/activate checking model more explicit, especially
since we now have intel_fbc_can_activate().

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-4-git-send-email-paulo.r.zanoni@intel.com