video: exynos_dp: add bit-masking for LINK_TRAINING_CTL register

This patch adds bit-masking for LINK_TRAINING_CTL register, when
pre-emphasis level is set. The bit 3 and bit 2 of LINK_TRAINING_CTL
register are used for pre-emphasis level setting, so other bits
should be masked.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>