drm: Describe dp_rev for drm_dp_set_phy_test_pattern

drivers/gpu/drm/drm_dp_helper.c:1598: warning: Function parameter or member 'dp_rev' not described in 'drm_dp_set_phy_test_pattern'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200516212330.13633-3-chris@chris-wilson.co.uk