msm: mdss: Extend GC programming to writeback mixers in pp

Currently pp_mixer_setup does not configure the gamma
correction in writeback mixers. Extending the function to
incorporate programming of writeback mixers as well.

Change-Id: I9c2ffa0153e642d603474965d6d0372dd6403685
Signed-off-by: Krishna Chaitanya Parimi <cparimi@codeaurora.org>