drm/amd/display: remove dc context from transfer function

[Why]
The ctx field of dc_transfer_func is not always populated and therefore
isn't reliable.

[How]
Remove dc context from dc_transfer_func

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>