drm/rockchip: fix spelling mistake "modifer" -> "modifier"

There is a spelling mistake in a DRM_DEBUG_KMS debug message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415083420.366279-1-colin.king@canonical.com