drm/i915: promote ddb update message to DRM_DEBUG_KMS

This message is currently marked as DRM_DEBUG_ATOMIC. I would like it
to be DRM_DEBUG_KMS since it is more KMS than atomic, and this will
also make the message appear in the CI logs, which may or may not help
us with some FIFO underrun bugs.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181004231600.14101-7-paulo.r.zanoni@intel.com