fate: Only generate tests/pixfmts.mak if some pixfmts fate test is run

tests/fate/filter-video.mak

@@ -122,7 +122,9 @@ tests/pixfmts.mak: avconv$(EXESUF)
 	$(Q)$(TARGET_EXEC) $(TARGET_PATH)/$< -pix_fmts list 2> /dev/null | awk 'NR > 8 && /^IO/ { printf $$2 " " }' >> $@
 	$(Q)printf "\n" >> $@
 
-ifneq (,$(RUNNING_FATE))
+RUNNING_PIXFMTS_TESTS := $(filter check fate-list fate-filter-pixdesc%,$(MAKECMDGOALS))
+
+ifneq (,$(RUNNING_PIXFMTS_TESTS))
 -include tests/pixfmts.mak
 endif