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.

src/gallium/drivers/v3d/v3d_simulator.c

@@ -580,6 +580,7 @@ v3d_simulator_init(struct v3d_screen *screen)
         sim_file->gmp = u_mmAllocMem(sim_state.heap, 8096, GMP_ALIGN2, 0);
         sim_file->gmp_vaddr = (sim_state.mem + sim_file->gmp->ofs -
                                sim_state.mem_base);
+        memset(sim_file->gmp_vaddr, 0, 8096);
 }
 
 void