msm_fb: MHL: Boot param support for enabling MHL on 8930

Added the boot params to specify primary and external displays.
The external display boot param can also be used to specify
whether MHL transmitter is enabled.

Change-Id: I3122774ab4b2bc60486b718522f0d69043b353c6
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>