intel/blorp_blit: Move RGB=>R conversion to follow blit splitting

intel/blorp_blit: Move RGB=>R conversion to follow blit splitting

In blorp_copy, when RGB surfaces are copied, we convert the
destination surface to a Red only surface, but 3 times as wide. This
introduces an implicit restriction of "mod 3" for the destination
width.

It is easier to handle the blorp split buffer offsetting with the
original RGB surface, and do the RGB=>R after this.

Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>