msm: kgsl: Add max and min power level controls in sysfs

Add min_pwrlevel, max_pwrlevel and thermal_pwrlevel to give a
privileged user more control over which power levels are considered
during DCVS power management.  max_pwrlevel is the maximum power
level that the system can go to at any time, min_pwrlevel represents
the lowest power level.  DCVS will chose any level between these
two extremes.  thermal_pwrlevel allows a daemon to set an absolute
top for frequency to prevent thermal issues. The effective maximum
power level is considered to be the lower of thermal or max.

Also added is num_pwrlevels that shows the number of active power
levels. This corresponds to the gpu frequencies in
available_gpu_frequencies.

Change-Id: Ic0dedbad20102d9a3c3350055e6fcd10358fc53d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>