v3d: Clear the GMP on initialization of the simulator.

v3d: Clear the GMP on initialization of the simulator.

Otherwise, we might have pages accessible that shouldn't be and miss out
on errors.  This is unlikely for most tests since v3d_hw_get_mem() is big
enough that it'll be a freshly zeroed mmap, but if screens are destroyed
and recreated then we'd be reusing the old v3d_hw_get_mem() contents.