drm/i915: move i915_map_type into i915_gem_object_types.h

Looks like it belongs there anyway, otherwise we have to include the
entirety of i915_gem_object.h just to get at the enum.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119133106.66294-3-matthew.auld@intel.com