vc4: Restructure the simulator mode.

vc4: Restructure the simulator mode.

Rather than having simulator mode changes scattered around vc4_bufmgr.c
and vc4_screen.c, make vc4_bufmgr.c just call a vc4_simulator_ioctl, which
then dispatches to a corresponding implementation.

This will give the simulator support a centralized place to do tricks like
storing most BOs directly in simulator memory rather than copying in and
out.

This leaves special casing of mmaping BOs and execution, because of the
winsys mapping.