drm/i915: Export intel_framebuffer_fini

Rather than open-code the teardown of a framebuffer, export the routine
from intel_display.c. This then make intel_fbdev symmetric in its use of
the common intel_framebuffer routines to initialise and clean up the
struct intel_framebuffer. (And new features need only be added in one
location!)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>