drm/amd/display: Fic incorrect pipe being used for clk update

[Why]
we save the prev_dppclk value using "dpp_inst" but
when reading this value we use the index "i". In
a case where a pipe is fused off we can end up reading
the incorrect instance because i != dpp_inst in this
case.

[How]
read the prev_dppclk using dpp_inst instead of i

Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>