drm: omapdrm: Fix race condition between GO and vblank IRQ

The vblank interrupt is used by the driver as a completion signal when
applying new settings.

A race condition exist between enabling the vblank interrupt and
applying new settings to the hardware by setting the GO bit. If a vblank
interrupt occurs in-between, the driver will incorrectly consider the
new settings to be applied. Fix this by enabling the interrupt after
setting the GO bit.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>