drm/amd/display: add dummy PG callback for beige goby

[Why&How]
PG registers are absent in beige goby, so programming these registers
generate call trace. Fix this by adding a dummy function in dcn303
initialization function which prevents writing to non existant registers.

Fixes: cd6d421e ("drm/amd/display: Initial DC support for Beige Goby")
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>