drm/sched: Define pr_fmt() for DRM using pr_*()

Define pr_fmt() as "[drm] " for DRM code using pr_*() facilities, especially
when no devices are available. This makes it easier to browse kernel logs.

Signed-off-by: Luben Tuikov <ltuikov89@gmail.com>
Acked-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110002659.113208-2-ltuikov89@gmail.com