drm/i915: Document locking guidelines

To ensure cross-driver locking compatibility, document the expected
guidelines for implementing the GEM locking in i915. Note that this
is a description of how things should end up after being reworked,
and does not reflect the current state of things.

v2: Use rst note:: tag (Rodrigo)

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: CQ Tang <cq.tang@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190830105053.17491-1-joonas.lahtinen@linux.intel.com