gfx: drv: Lock gPVRSRVLock mutex before touching the meminfo/bm ref counts

gfx: drv: Lock gPVRSRVLock mutex before touching the meminfo/bm ref counts

The pvr mem ref counts are not modified using atomic operations, so we
need to protect them against simultaneous RMW access. Additionally it's
not at all clear how the related data structures are protected in the
pvr code. Use gPVRSRVLock as that's always locked around bridge ioctls.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>