drm/tegra: gem: Open code drm_prime_gem_destroy

Since we aren't using drm_gem_prime_import(_dev), for clarity don't
use the corresponding destroy function either.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240424051335.2872574-1-cyndis@kapsi.fi