drm/amd/display: Fix DCN35 set min dispclk logic

[Why]

Setting min dispclk to 50Mhz outside clock lowering function causes
unnecessary calls to SMU to lower dispclk and causes dentist hangs when
there is no stream on the pipes.

[How]

Move the set minimum dispclk logic inside the lowering dispclk if
statement.

Fixes: 23444132 ("DCN35 set min dispclk to 50Mhz")
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Nicholas Susanto <Nicholas.Susanto@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>