drm/amd: Tighten permissions on VBIOS flashing attributes

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>