drm/i915: use DRM_ERROR() instead of drm_err()

Unify on current common usage to allow repurposing drm_err() later. Fix
newlines while at it.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002145405.27848-1-jani.nikula@intel.com