i965/blorp: Remove unnecessary test in gen7_blorp_emit_depth_stencil_config.

gen7_blorp_emit_depth_stencil_config() is only called when
params->depth.mt is non-null.  Therefore, it's not necessary to do an
"if (params->depth.mt)" test inside it.  The presence of this if test
was misleading static analysis tools (and briefly, me) into thinking
that gen7_blorp_emit_depth_stencil_config() might sometimes access
uninitialized data and dereference a null pointer.