drm/i915/fbc: extract intel_fbc_can_activate()

Extract all the code that checks if the FBC configuration is valid to
its own function, making __intel_fbc_update() much simpler.

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/1453210558-7875-3-git-send-email-paulo.r.zanoni@intel.com