drm/amd/display: change default pipe_split policy for DCN1

[Why]
Changing policy to dynamic will allow 4k multi display configs
to be supported at DPM0

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>