msm: mdss: add panel parameters to debugfs

Export panel properties in debugfs nodes. Panel property values
can be get and set by reading and writing debugfs nodes, which
helps panel bring up and debug instead of changing dtsi files.

Change-Id: I2c658c4bf2a0f0c0713df0ab8898380227f0a03b
Signed-off-by: Xu Yang <yangxu@codeaurora.org>