drm/amdkfd: Use correct major in devcgroup check

The existing code used the major version number of the DRM driver
instead of the device major number of the DRM subsystem for
validating access for a devices cgroup.

This meant that accesses allowed by the devices cgroup weren't
permitted and certain accesses denied by the devices cgroup were
permitted (if they matched the wrong major device number).

Signed-off-by: Lorenz Brun <lorenz@brun.one>
Fixes: 6b855f7b ("drm/amdkfd: Check against device cgroup")
Reviewed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org