i965/blorp: Correct type of src_format in call to intel_miptree_texture_aux_usage

i965/blorp: Correct type of src_format in call to intel_miptree_texture_aux_usage

intel_miptree_texture_aux_usage() takes an isl_format, but we are
passing a mesa_format. clang warns:

 brw_blorp.c:305:52: warning: implicit conversion from enumeration
    type 'mesa_format' to different enumeration type
    'enum isl_format' [-Wenum-conversion]
       intel_miptree_texture_aux_usage(brw, src_mt, src_format);
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~              ^~~~~~~~~~

Fixes: fc1639e46d ("i965/blorp: Use texture/render_aux_usage for blits")
Cc: "17.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>