msm: mdss: Avoid dsi v2 compilation for ferrum

Ferrum uses DSI 6G, and there is no need to compile dsi v2
driver for this target. Update Makefile to take care of this.

Change-Id: Ifbbb320fd908bbdca5825cac7164b68b18360316
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>