video: fbdev: imxfb: remove the macros for initializing the DMACR

The current definitions of DMACR_HM() and DMACR_TM() are correct only
for imx1, they're wrong for imx21.

The macros are meant for legacy board files only, they're not applicable
for boards using device tree.

At the moment, there are no boards using these macros. So it should be
safe to drop them rather than making them work for both imx1 and imx21,
which would require a change in the platform data struct.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>