drm/i915: Fix i915_gem_context.h header

Header uses I915_NUM_ENGINES so needs to include i915.gem.h, and also
it uses requests so we can forward declare struct drm_i915_gem_request.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180213141833.17012-1-tvrtko.ursulin@linux.intel.com