drm/debugfs: fix potential NULL pointer dereference

encoder->funcs entry might be NULL, so check it first before calling its
methods. This fixes NULL pointer dereference observed on Rasberry Pi
3b/4b boards.

Fixes: caf525ed ("drm/encoder: register per-encoder debugfs dir")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205130631.3456986-1-m.szyprowski@samsung.com