drm/tegra: Add SET/GET_FLAGS IOCTLs

The DRM_TEGRA_GEM_SET_FLAGS IOCTL can be used to set the flags of a
buffer object after it has been allocated or imported. Flags associated
with a buffer object can be queried using the DRM_TEGRA_GEM_GET_FLAGS
IOCTL.

Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>