msm: mdss: Fix gamut mapping read issue

When attempting to read gamut mapping data from the hardware, the gamut
LUTs were read incorrectly because the LUT index was not reset to 0
before reading. Furthermore, the data read from the register was not
masked correctly before being copied to user.

Change-Id: Ic8a99d7987578fa959fa1b5fd5f1ec6966e60409
Signed-off-by: Benet Clark <benetc@codeaurora.org>