drm/i915/fbc: Limit cfb to the first 256MiB of stolen on g4x+

drm/i915/fbc: Limit cfb to the first 256MiB of stolen on g4x+

Since g4x the CFB base only takes a 28bit offset into stolen.
Not sure if the CFB is allowed to start below that limit but
then extend beyond it. Let's assume not and just restrict the
allocation to the first 256MiB (in the unlikely case
we have more stolen than that).

v2: s/BIT/BIT_ULL/ (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200714201945.18959-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>