drm/amd/display: Rename dcn31 string shown to user

[Why]
DCN31 isn't a product, but DCN312 is. Matching against documentation users
might not understand the code.

[How]
Change DCN 3.1 string to be DCN 3.1.2.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>