msm: kgsl: Disable SP/TP power collapse

Simplify the control path for the SP/TP power collapse feature.
Make the sysfs variable "sptp_pc" and write 1 to enable power collapse,
0 to disable.  Explicitly list the feature for adreno cores where it
is enabled.

Change-Id: I9a47b7a62013d74944df1f478240ae66c2db742f
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>