msm: VPU: Add power frequency mode support

The VPU supports three power modes: SVS, Nominal and Turbo. When in
dynamic mode, VPU driver selects the best power frequency mode for the
current VPU configuration. This change implements that support while
allowing to force one specific power mode through debugfs.

Change-Id: I4575247140f972f84f36b8b6935831f6c4bbba16
Signed-off-by: Stephane Viau <sviau@codeaurora.org>