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.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>