gpu: ipu-v3: ipu-dc: Remove unused 'di' variable

The 'di' variable is never used inside ipu_dc_enable_channel(),
so just remove it.

This fixes the following build warning with W=1:

drivers/gpu/ipu-v3/ipu-dc.c: In function 'ipu_dc_enable_channel':
drivers/gpu/ipu-v3/ipu-dc.c:252:6: warning: variable 'di' set but not used [-Wunused-but-set-variable]

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>