drm/tegra: Rename host1x_drm_file to tegra_drm_file

This structure extends drm_file with Tegra DRM specific fields and has
nothing to do with host1x. Rename the structure to more clearly mark the
boundaries between host1x and Tegra DRM.

While at it, move the structure definition out of the header. It's never
used outside of the drm.c source file, so it can be defined within that.

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