drm/amd/display: Avoid avoid unnecessary pixel rate divider programming

[Why]
Programming pixel rate divider when FIFO is enabled can cause FIFO error.

[How]
Skip divider programming when divider values are the same to prevent FIFO
error.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Taimur Hassan <Syed.Hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>