MAINTAINERS: add Documentation/gpu and Documentation/gpu/i915.rst

We'll want to keep an eye on what's going on in these files.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/4d409e09c475cc0bdf7a0312e30c0d3f8d535fc5.1466506505.git.jani.nikula@intel.com