drm/drm_file: fix use of uninitialized variable

smatch reports:

drivers/gpu/drm/drm_file.c:967 drm_show_memory_stats() error: uninitialized symbol 'supported_status'.

'supported_status' is only set in one code path. I'm not familiar with
the code to say if that path will always be ran in real life, but
whether that is the case or not, I think it is good to initialize
'supported_status' to 0 to silence the warning (and possibly fix a bug).

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231103-uninit-fixes-v2-1-c22b2444f5f5@ideasonboard.com