All DRM_CLIENT capabilities are tied to KMS support, so returning
-EOPNOTSUPP when KMS is not supported.

v2: returning -EOPNOTSUPP(same value as posix ENOTSUP and available
in uapi) instead of -ENOTSUPP

v3: adding comments about the feature requirement about capabilities

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180918174809.17123-1-jose.souza@intel.com