i965/blorp: Don't fall back to swrast when miptrees absent.

i965/blorp: Don't fall back to swrast when miptrees absent.

Previously, the blorp engine would fall back to swrast if the source
or destination of a blit had no associated miptree.  This was
unnecessary, since _mesa_BlitFramebufferEXT() already takes care of
making the blit silently succeed if there are no buffers bound, so the
fallback paths could never actually happen in practice.

Removing these fallback paths will simplify the implementation of
correct DrawBuffers support in blorp.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>