drm/i915/execlists: Move RPCS setup to context pin

drm/i915/execlists: Move RPCS setup to context pin

Configuring RPCS in context image just before pin is sufficient and will
come extra handy in one of the following patches.

v2:
 * Split image setup a bit differently. (Chris Wilson)

v3:
 * Update context image after reset as well - otherwise the application
   of pinned default state clears the RPCS.

v4:
 * Use local variable throughout the function. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125023005.1007-1-chris@chris-wilson.co.uk