drm/tegra: fb: Implement ->fb_mmap() callback

This fixes hangs with legacy applications that use the mmap() syscall on
the fbdev device to map framebuffer memory. The fbdev implementation for
mmap() creates a mapping that conflicts with DRM usage and causes a hang
when the memory is accessed through the mapping.

Reported-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Tested-by: Stefan Agner <stefan@agner.ch>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reported-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>