drm/i915/selftests: Exercise all copy engines with the blt routines

Just to remove an obnoxious HAS_ENGINES(), and in the process make the
code agnostic to the availabilty of any particular engine by making it
exercise any and all such engines declared on the system.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200604123641.767-1-chris@chris-wilson.co.uk