drm/i915/gt: limit lmem allocation size to succeed on SmallBars

In case system is short on mappable memory (256MB on SmallBar) allocation
of two 1GB buffers will fail.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/8300


Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230601144450.792228-1-andrzej.hajda@intel.com