drm/amd/display: initialize dc_transfer_func->ctx

[Why]
dc_transfer_func structs were being passed around with a null
pointer, waiting for unsuspecting programmers to dereference it.

[How]
Initialize it

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>