gpu: drm: sti: fix spelling mistake: "initialze" -> "initialize"

Trivial fix to spelling mistake in DRM_ERROR error message

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/20180502101048.8442-1-colin.king@canonical.com