drm/amd/display: Use seperate dmcub firmware for navy_flounder

[Why]
Currently navy_flounder is using sienna_cichlid_dmcub.bin.

[How]
Create a seperate define so navy_flounder will use its own firmware.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>