msm_fb: MHL: add fwd declarations

If MHL feature is enabled from kernel config compiler throws errors.
This is due to the re-ordering of the function definition in some of
the earlier patches. Need forward declarations of a few functions to fix
compilation issues.

Change-Id: I486b3d6efc33f6557b5ef301bd7037867bb8c562
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>