msm: mdss: Reading IGC and PGC LUT from cached values

Certain targets like msm8226 do not have capability of reading
IGC and PGC LUT from hardware registers.
Adding functions to read the LUT values from software cache.

Change-Id: Ifd0f5f131b7ff1fd7d1fb122425241a0e0cec328
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>