host1x: Fallback to fullscreen if overlay not supported

If hardware overlays are not supported, fallback to using fullscreen
mode.

Signed-off-by: Thierry Reding <thierry@gilfi.de>
host1x: Make host1x_bo_mmap() more usable

Since host1x_bo is an opaque type, the pointer to the mmap()'ed region
isn't accessible to users. To achieve this, make the host1x_bo_mmap()
function return the pointer in an output argument.

Signed-off-by: Thierry Reding <thierry@gilfi.de>