drm: imx: convert to drm_crtc_send_vblank_event()

ipu_crtc_handle_pageflip() was calling drm_send_vblank_event() with
a pipe argument of -1.  Commit cc1ef118 ("drm/irq: Make pipe
unsigned and name consistent") now makes this error obvious, as we
now may get a warning from:

	if (WARN_ON(pipe >= dev->num_crtcs))

in drm_vblank_count_and_time().  Prior to this change, we would end
up making out-of-bounds array accesses via:

	struct drm_vblank_crtc *vblank = &dev->vblank[crtc];
and
	*vblanktime = vblanktimestamp(dev, pipe, cur_vblank);

So, this has been broken for a very long time, and is not a result
of the above commit.  Since we don't care about the staging versions,
I've tagged this with the earliest mainline commit where we do care,
even though this commit did not introduce the bug.

Fixes: 6556f7f8

 ("drm: imx: Move imx-drm driver out of staging")
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>