gpu: nvgpu: gk20a: Update perfmon init

gpu: nvgpu: gk20a: Update perfmon init

Make the perfmon sampling configurable, by adding an 'enabled' flag.
This is set according to the CONFIG initially. Modify the perfmon event
handler to not touch clock rates. Add a counter to count the number of
perfmon events.

Also add debugfs entries for the above.

Bug 1410515

Change-Id: Ic8197eef0e46e35af1179a5b06140393541cfd43
Signed-off-by: Prashant Malani <pmalani@nvidia.com>
Reviewed-on: http://git-master/r/351564
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>