drm/panthor: Restrict high priorities on group_create

We were allowing any users to create a high priority group without any
permission checks. As a result, this was allowing possible denial of
service.

We now only allow the DRM master or users with the CAP_SYS_NICE
capability to set higher priorities than PANTHOR_GROUP_PRIORITY_MEDIUM.

As the sole user of that uAPI lives in Mesa and hardcode a value of
MEDIUM [1], this should be safe to do.

Additionally, as those checks are performed at the ioctl level,
panthor_group_create now only check for priority level validity.

[1]https://gitlab.freedesktop.org/mesa/mesa/-/blob/f390835074bdf162a63deb0311d1a6de527f9f89/src/gallium/drivers/panfrost/pan_csf.c#L1038

Signed-off-by: Mary Guillemard <[email protected]>
Fixes: de8548813824 ("drm/panthor: Add the scheduler logical block")
Cc: [email protected]
Reviewed-by: Boris Brezillon <[email protected]>
Signed-off-by: Boris Brezillon <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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