drm/tegra: Fix non-debugfs builds

The new debugfs registration fails to build when CONFIG_DEBUGFS is
disabled, because the drm_crtc structure is lacking a member in that
configuration:

drivers/gpu/drm/tegra/dc.c: In function 'tegra_dc_late_register':
drivers/gpu/drm/tegra/dc.c:1204:28: error: 'struct drm_crtc' has no member named 'debugfs_entry'

Without CONFIG_DEBUGFS, the rest of the function already degrades
into nothing, so we just avoid the one assignment.

Fixes: b95800ee ("drm/tegra: dc: Register debugfs in ->late_register()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>