drm/encoder: Don't raise voice in drm_encoder_mask() documentation

There's no need to raise our voice when saying encoder, we're all
civilized adults here!

Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913222704.8241-4-lyude@redhat.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>