drm/msm/dp: fix incorrect function prototype of dp_debug_get()

Fix the incorrect function prototype for dp_debug_get()
in the dp_debug module to address compilation warning.
Also add prototype for msm_dp_debugfs_init() for fixing compilation
issue with other defconfigs.

changes in v2:
	- add prototype for msm_dp_debugfs_init()

Fixes: f913454a ("drm/msm/dp: move debugfs node to /sys/kernel/debug/dri/*/")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>