msm: kgsl: report an error if gfx power rail cannot be turned on

Make sure we return an error to the caller if the power rail cannot
be turned on during gpu start.

Change-Id: Ifeead23d7dd1664ce172a8be3e0a2215350b0839
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>