gfx: pvr: Dereference a pointer after null check

gfx: pvr: Dereference a pointer after null check

Dereferencing a pointer may only happen after NULL check to avoid crash
if the pointer is NULL.

[pvr/services4/srvkm/common/buffer_manager.c:1152]: (error) Possible
null pointer dereference: psBMHeap - otherwise it is redundant to check
if psBMHeap is null at line 1156

Reported-by: Olev Kartau <olev.kartau@intel.com>
Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>