drm/nouveau/nvif: remove const attribute from nvif_mclass

Similar to commit 0bf8bf50

 ("module: Remove
const attribute from alias for MODULE_DEVICE_TABLE")

Fixes many -Wduplicate-decl-specifier warnings due to the combination of
const typeof() of already const variables.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>