drm/amdgpu: add one definition for RAS's sysfs/debugfs name(v2)

Add one definition for the RAS module's FS name. It's used
in both debugfs and sysfs cases.

v2: Use static variable instead of macro definition.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>