drm: fixes when i2c encoder slave mode_fixup is null.

Avoid i2c slave encoder drivers to copy/paste code to implement functions that will
only return true.

Signed-off-by: Carlos Palminha <palminha@synopsys.com>
[danvet: whitespace requested by Jani.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455106118-32145-1-git-send-email-palminha@synopsys.com

drivers/gpu/drm/drm_encoder_slave.c

@@ -140,6 +140,9 @@ bool drm_i2c_encoder_mode_fixup(struct drm_encoder *encoder,
 		const struct drm_display_mode *mode,
 		struct drm_display_mode *adjusted_mode)
 {
+	if (!get_slave_funcs(encoder)->mode_fixup)
+		return true;
+
 	return get_slave_funcs(encoder)->mode_fixup(encoder, mode, adjusted_mode);
 }
 EXPORT_SYMBOL(drm_i2c_encoder_mode_fixup);