drm/amd/display: Fix typo when retrieving dppclk from UEFI config

[why]
In some boot configurations we need to retrieve the currently
UEFI-set dppclk, but there was a typo in the calculation

[how]
Fix typo to make dpp_clk calculate off dpp_clk divider instead of
disp_clk

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Martin Leung <martin.leung@amd.com>
Reviewed-by: Sung Lee <Sung.Lee@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>