drm/msm/sde: Fix gamma correction table indexing

Gamma correction table index was incorrectly set which will lead to
color swap issue. Change fixes the index calculation and uses the reg
dma method to set the opcode/swap registers.

Change-Id: Ia0bee41c913108dcb4e70c0dd0701aa0a6bee904
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>