drm/i915: determine lmem_size properly

Determine lmem_size using ADDR_RANGE register so that lmem_setup()
works on platform with small-bar as well.

Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220511153746.14142-2-nirmoy.das@intel.com