disp: msm: dsi: add debug support to configure clock gating

Add support to selectively enable clock gating for supported
DSI clocks using a new debugfs node - config_clk_gating. This
new node would be created for every display node. See below
for usage examples:

To enable clock gating only for BYTE clock:
echo 1 > /sys/kernel/debug/<display_name>/config_clock_gating

To enable clock gating only for PIXEL clock:
echo 2 > /sys/kernel/debug/<display_name>/config_clock_gating

To enable clock gating only for PHY clock:
echo 4 > /sys/kernel/debug/<display_name>/config_clock_gating

To enable clock gating only for all clock:
echo 7 > /sys/kernel/debug/<display_name>/config_clock_gating

To disable clock gating for all clocks:
echo 8 > /sys/kernel/debug/<display_name>/config_clock_gating

To go back to default setting:
echo 0 > /sys/kernel/debug/<display_name>/config_clock_gating

Change-Id: I83713d86eb1b9675d40d51fc20de81cca0aeb1c0
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>