msm_fb: display: Enable display debugging through mdp debugfs

Change the config from DEBUG_FS to MDP_DEBUG_FS to dump and
write the MDP, MDDI and HDMI debug registers. By default
CONFIG_MDP_DEBUG_FS should be disabled and can be enabled
through defconfig file.

Change-Id: I2ed8dcc30b19a80912734ec13f24a67351c38315
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>
BUG=26404525