gpu: pvr: add pvr_lock/remove unneeded lock headers

gpu: pvr: add pvr_lock/remove unneeded lock headers

- Add a public interface for accessing this lock. It has been used
  as a public interface anyway and an upcoming patch needs to access
  it as well.

- Remove headers becoming unnecessary by this refactoring.

Note:
  Currently this lock provides for a course level locking protecting
  all HW and SW state tracking objects. We'll need to revise the use
  of the following additional locks in the driver. Their existance
  might not be justified, in which case we could just substitute
  them with pvr_lock:

  PVRSRVDvfsLock
  PVRSRVPowerLock

Signed-off-by: Imre Deak <imre.deak@nokia.com>