drm/tegra: Rename host1x_drm_context to tegra_drm_context

The structure represents a context associated with a particular process
that has opened the Tegra DRM device and requested a channel. This is a
very DRM-specific notion and has nothing to do with host1x. Rename the
structure to more clearly mark the boundaries between the two.

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