drm/imx: Don't set a gamma table size

imx doesn't have any functions for setting the gamma table, so this is
completely defunct.

Not nice to lie to userspace, so let's stop!

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>