mdss: display: remove the pwm gpio request from edp backlight

There is no use of the pwm gpio inside the driver so cleaning up the code
and removing it from devicetree.

Change-Id: I72c038f3082aa6ed18ea9142de75faecc1e1e0fd
Signed-off-by: Asaf Penso <apenso@codeaurora.org>