drm/i915: Convert uncleared FIFO underrun message to errors

Some platforms have a shared error interrupt, so if FIFO underrun
reporting gets disabled for one pipe/transcoder it gets disabled
for all pipes/transcoders.

When we disable FIFO underrun reporting we check whether the
interrupt was enabled or not. If it wasn't we might have missed
an underrun and we perform one last check right there. Currently
we print a debug message when an underrun is detect using this
mechanism. Promote the message to DRM_ERROR() to match the other
underrun error messages.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>