disp: msm: modify handling of debugfs initialization

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

Change-Id: Iaeaf51b3654c9458cf8131a9756e6b905007c4ae
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>
Signed-off-by: Samantha Tran <samtran@codeaurora.org>