staging: imx-drm: Fix probe failure

Since commit b5dc0d10 (drm/imx: kill firstopen callback) the following probe
failure is seen:

[drm] Supports vblank timestamp caching Rev 1 (10.10.2010).
[drm] No driver support for vblank timestamp query.
[drm] Initialized imx-drm 1.0.0 20120507 on minor 0
imx-ldb ldb.10: adding encoder failed with -16
imx-ldb: probe of ldb.10 failed with error -16
imx-ipuv3 2400000.ipu: IPUv3H probed
imx-ipuv3 2800000.ipu: IPUv3H probed
imx-ipuv3-crtc imx-ipuv3-crtc.0: adding crtc failed with -16.
imx-ipuv3-crtc: probe of imx-ipuv3-crtc.0 failed with error -16
imx-ipuv3-crtc imx-ipuv3-crtc.1: adding crtc failed with -16.
imx-ipuv3-crtc: probe of imx-ipuv3-crtc.1 failed with error -16
imx-ipuv3-crtc imx-ipuv3-crtc.2: adding crtc failed with -16.
imx-ipuv3-crtc: probe of imx-ipuv3-crtc.2 failed with error -16
imx-ipuv3-crtc imx-ipuv3-crtc.3: adding crtc failed with -16.
imx-ipuv3-crtc: probe of imx-ipuv3-crtc.3 failed with error -16

The reason for the probe failure is that now 'imxdrm->references' is incremented
early in imx_drm_driver_load(), so the following checks in imx_drm_add_crtc()
and imx_drm_add_encoder():

	if (imxdrm->references) {
		ret = -EBUSY;
		goto err_busy;
	}

,will always fail.

Instead of manually keeping the references in the imx-drm driver, let's use
drm->open_count.

After this patch, lvds panel is functional on a mx6qsabrelite board.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>