drm/i915/display: Implement FBC support

Initialise the stolen memory allocator earlier to make it available
when FBC is initialised, so we can check if it's succesfull there.

With this, we can easily implement FBC by hooking a few calls
inside intel_fbc.c

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>