drm/panthor: Fix NULL vs IS_ERR() bug in panthor_probe()

The devm_drm_dev_alloc() function returns error pointers.
Update the error handling to check for error pointers instead of NULL.

Fixes: 4bdca115 ("drm/panthor: Add the driver frontend block")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402104041.1689951-1-harshit.m.mogalapalli@oracle.com