drm/fb_helper: Fix checkpatch errors

Fixes the following checkpatch errors:
ERROR: trailing statements should be on next line
98: FILE: gpu/drm/drm_fb_helper.c:98:
	case DRM_FORCE_OFF: s = "OFF"; break;
ERROR: trailing statements should be on next line
99: FILE: gpu/drm/drm_fb_helper.c:99:
	case DRM_FORCE_ON_DIGITAL: s = "ON - dig"; break;
ERROR: trailing statements should be on next line
101: FILE: gpu/drm/drm_fb_helper.c:101:
	case DRM_FORCE_ON: s = "ON"; break;

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>