OMAPDSS:DISPC: Gamma feature implementation

I think this code needs more work: beta in leanKernel for now.

This patch enables the gamma table to be loaded. Gamma value
can range from 0.2 to 2.0 in steps of 0.2 and thus they have
indices from 1 to 10 correspondingly. To disable gamma, value
shall be set to zero. The different values of gamma can also
be used to alter the brightness level of image.

Change-Id: I212a33f5b87c6510ba36720594378affa9ed5bba
Signed-off-by: Sujeet Barnawal <s-baranwal@ti.com>
gamma control: allow sysfw write access to all
omapdss: remove warnings in gamma code