drm/tegra: Remove unneeded include

The header file 'tegra_drm.h' is included in gem.c, but this file is
already include 'drm.h'. Although there is no harm in including this
file again, it is also not necessary. Furthermore, the header file is
located under 'include/uapi/drm' so ideally the full path would be
used to be explicit. For now, just remove from gem.c.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20250428153435.1013101-1-jonathanh@nvidia.com