MAINTAINERS: Update DRM GPU Scheduler section

Luben has not been active and has not responded to mails since summer
2024. Remove him from MAINTAINERS and add an entry in CREDITS.

Philipp has a new email address and an ACK to commit work time to the
scheduler. Thus, set the state to 'Supported'.

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250113121851.31382-1-phasta@kernel.org