drm/i915/gem: Zap the i915_gem_object_blt code

It's unused with the exception of selftest. Replace a call in the
memory_region live selftest with a call into a corresponding
function in the new migrate code.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617063018.92802-13-thomas.hellstrom@linux.intel.com