drm/i915/fbc: extract intel_fbc_can_enable()

Extract that part of the code to a new function and call this function
only once during intel_fbc_choose_crtc() instead of once per plane.
Those checks are independent from planes/CRTCs.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1478883461-20201-4-git-send-email-paulo.r.zanoni@intel.com