drm/tegra: gem: Make __tegra_gem_mmap() available more widely

This function allows mapping a GEM object into a virtual memory address
space, which makes it useful outside of the GEM code.

While at it, rename the function so it doesn't clash with the function
that implements the DRM_TEGRA_GEM_MMAP IOCTL.

Signed-off-by: Thierry Reding <treding@nvidia.com>