drm/omap: fix primary-plane's possible_crtcs

We set the possible_crtc for all planes to "(1 << priv->num_crtcs) - 1",
which is fine as the HW planes can be used fro all crtcs. However, when
we're doing that, we are still incrementing 'num_crtcs', and we'll end
up with bad possible_crtcs, preventing the use of the primary planes.

This patch passes a possible_crtcs mask to plane init function so that
we get correct possible_crtc.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>