[Why]
DCE6 does not have CURSOR2_DEGAMMA_MODE bit in DEGAMMA_CONTROL register

[How]
Add DCE6 specific macros definitions for IPP masks
DCE6 IPP macros will avoid buiding errors when using DCE6 headers
Add dce60_ipp_set_degamma() function w/o Cursor2 Degamma programming
Use dce60_ipp_set_degamma() in ipp_funcs dce60_ipp_funcs
Add DCE6 specific dce60_ipp_construct
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>