drm/amd/display: Don't overclock DCE 6 by 15%

The extra 15% clock was added as a workaround for a Polaris issue
which uses DCE 11, and should not have been used on DCE 6 which
is already hardcoded to the highest possible display clock.
Unfortunately, the extra 15% was mistakenly copied and kept
even on code paths which don't affect Polaris.

This commit fixes that and also adds a check to make sure
not to exceed the maximum DCE 6 display clock.

Fixes: 8cd61c31 ("drm/amd/display: Raise dispclk value for Polaris")
Fixes: dc88b4a6 ("drm/amd/display: make clk mgr soc specific")
Fixes: 3ecb3b79 ("drm/amd/display: dc/clk_mgr: add support for SI parts (v2)")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Rodrigo Siqueira <siqueira@igalia.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>