msm: kgsl: Provide a sysfs interface to force GPU power rail on

Provide a debugging interface to keep power rail permanently
enabled. Setting the force_rail_on sysfs entry will leave the
power rail always on.

Change-Id: I10a69dbf5b8cf2e2673b0fcc5e5fb5500a388904
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>