drm/amd: Tighten permissions on VBIOS flashing attributes

commit 7ab1a4913d0051cf5196ef7987b5fa42c25e13b6 upstream.

Non-root users shouldn't be able to try to trigger a VBIOS flash
or query the flashing status.  This should be reserved for users with the
appropriate permissions.

Cc: stable@vger.kernel.org
Fixes: 8424f2cc

 ("drm/amdgpu/psp: Add vbflash sysfs interface support")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>