drm/msm: add calls to prepare and unprepare panel

Prepare the panel before it's enabled and un-prepare after disable, this
will make sure that the regulators are switched on and off correctly.

Tested it on APQ8064 based IFC6410 with panel.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>