disp: msm: dsi: modify handling of debugfs initialization

Add support to allow creation of debugfs node only if
CONFIG_DEBUG_FS is enabled.

Change-Id: I1ae2c4188a99e3ed88f59fc021efc01407bf942d
Signed-off-by: Harigovindan P <harigovi@codeaurora.org>
[rshaha@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Rahul Shahare <rshaha@codeaurora.org>