i965: automake: correctly set MKDIR_GEN

Otherwise we might end up w/o the respective folder (depending on
autotools version) and fail at build time.

Fixes: bfd17c76c12 "i965: Port INTEL_PRECISE_TRIG=1 to NIR."
Cc: "12.0 13.0" <mesa-dev@lists.freedesktop.org>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 5eed48d23780e812ca5c53fecc59a419962c7dd6)
diff --git a/src/mesa/drivers/dri/i965/Makefile.am b/src/mesa/drivers/dri/i965/Makefile.am
index 4b00977..92cb5b5 100644
--- a/src/mesa/drivers/dri/i965/Makefile.am
+++ b/src/mesa/drivers/dri/i965/Makefile.am
@@ -45,6 +45,7 @@
 
 AM_CXXFLAGS = $(AM_CFLAGS)
 
+MKDIR_GEN = $(AM_V_at)$(MKDIR_P) $(@D)
 brw_nir_trig_workarounds.c: brw_nir_trig_workarounds.py $(top_srcdir)/src/compiler/nir/nir_algebraic.py
 	$(MKDIR_GEN)
 	$(AM_V_GEN) PYTHONPATH=$(top_srcdir)/src/compiler/nir $(PYTHON2) $(PYTHON_FLAGS) $(srcdir)/brw_nir_trig_workarounds.py > $@ || ($(RM) $@; false)