drm/i915: extract gem/i915_gem_stolen.h from i915_drv.h

It used to be handy that we only had a couple of headers, but over time
i915_drv.h has become unwieldy. Extract declarations to a separate
header file corresponding to the implementation module, clarifying the
modularity of the driver.

Ensure the new header is self-contained, and do so with minimal further
includes, using forward declarations as needed. Include the new header
from i915_drv.h to avoid sprinkling includes all over the place; this
can be changed as a follow-up if necessary.

No functional changes.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0141b4e1f1bf2deb65730ce6973863a3a16ab38f.1565271681.git.jani.nikula@intel.com