gpu: pvr: fix locking on the DVFS path

gpu: pvr: fix locking on the DVFS path

The HW state is protected by pvr_lock(), so acquire it for the
duration of clock rate changes. We can also remove PVRSRVDvfsLock()
that thus becomes unneded.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
CC: Mark Underwood <mark.underwood@imgtec.com>
CC: Mark Riding <mark.riding@imgtec.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@nokia.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@nokia.com>