drm/i915/gt: Pull sseu context updates under gt

Lift the routines to emit a request to update the SSEU on the
intel_context out of i915_gem_context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200131104548.2451485-6-chris@chris-wilson.co.uk