drm/i915/stolen: Switch from DEBUG_KMS to DEBUG_DRIVER

i915_gem_stolen is an allocator for the reserved portion of memory
("stolen" from the system by the BIOS). It is not tied to KMS but
central to the driver, so prefer DRM_DEBUG_DRIVER.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180312165206.31772-1-chris@chris-wilson.co.uk