drm: Extract struct drm_mm_scan from struct drm_mm

The scan state occupies a large proportion of the struct drm_mm and is
rarely used and only contains temporary state. That makes it suitable to
moving to its struct and onto the stack of the callers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
[danvet: Fix up etnaviv to compile, was missing a BUG_ON.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>