msm: mdss: Add data mask when writing Gamut Mapping data

The gamut mapping data does only occupies 13 bits of the register.
Therefore, we should force the data that is written to be masked to
be the least significant 13 bits.

Change-Id: I37503ca3a46d9cb2cb1692763f301ae1d10f9d28
Signed-off-by: Benet Clark <benetc@codeaurora.org>