i965/blorp: Make the guts of brw_blorp_blit_miptrees miptree-unaware

i965/blorp: Make the guts of brw_blorp_blit_miptrees miptree-unaware

Now that we have the brw_blorp_surf struct, we can start to make bits of
blorp completely miptree-unaware.  To start things off, we split the guts
of brw_blorp_blit_miptrees into a brw_blorp_blit function which knows
nothing about miptrees.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>