drm/amd/display: Fix typo in get_pixel_rate_div

[Why & How]
Some FIFO errors still occur due to reading wrong pixel rate divider.
Fix typo 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>