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 <[e-mail]>
[danvet: whitespace requested by Jani.]
Signed-off-by: Daniel Vetter <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_encoder_slave.c

···
 		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);