mm-drivers: add support for build.sh techpack display_tp

add environment variable to be used by display-techpack.mk
for build.sh techpack display_tp.

Change-Id: I46b0ac3fb40371e3282191c75a501230243d9f52
Signed-off-by: Ashwin Pillai <[email protected]>

mm_driver_product.mk

@@ -13,3 +13,5 @@ ifeq ($(MM_DRV_DLKM_ENABLE), true)
 		PRODUCT_PACKAGES += sync_fence.ko msm_hw_fence.ko
 	endif
 endif
+
+DISPLAY_MM_DRIVER := msm_ext_display.ko sync_fence.ko msm_hw_fence.ko