staging: imx/drm: request irq only after adding the crtc

staging: imx/drm: request irq only after adding the crtc

If the bootloader already enabled the display, the interrupt handler
will be called as soon as it is registered. If the CRTC is not already
added at this time, the call to imx_drm_handle_vblank will result in
a NULL pointer dereference.

The patch fixes a kernel panic [1], which has been on linux-next since
Jan 8 [2].

[1] http://thread.gmane.org/gmane.linux.ports.arm.kernel/218858
[2] http://thread.gmane.org/gmane.linux.ports.arm.kernel/208192

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>