x86/gpu: Sprinkle const, __init and __initconst to stolen memory quirks

gen8_stolen_size() is missing __init, so add it.

Also all the intel_stolen_funcs structures can be marked
__initconst.

intel_stolen_ids[] can also be made const if we replace the
__initdata with __initconst.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>