drm/amd/display: Change Min fclk to 1.2Ghz

[Why]
Some nightly tests are failing since the new value for fclk is a
bit too low. Also, a new test for the maximum downscale case was
needed.

[How]
Updated the default value for fclk to be 1.2GHz.

Signed-off-by: Tyler DiBattista <tyler.dibattista@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>