disp: added environment variable for build.sh techpack display_tp

add a environment variable to build msm_drm for build.sh techpack
display_tp.

Change-Id: I530c4fef3c2bf6aa06f87ca3df46d1f155a91f3a
Signed-off-by: Ashwin Pillai <[email protected]>

display_driver_product.mk

@@ -10,3 +10,5 @@ endif
 ifeq ($(DISPLAY_DLKM_ENABLE),  true)
 	PRODUCT_PACKAGES += msm_drm.ko
 endif
+
+DISPLAY_MODULES_DRIVER := msm_drm.ko