drm/amd/display: Add dtbclk access to dcn315

[Why & How]

Currently DCN315 clk manager is missing code to enable/disable dtbclk.
Because of this, "optimized_required" flag is constantly set
and this prevents FreeSync from engaging for certain high bandwidth
display Modes which require DTBCLK.

Reviewed-by: Dmytro Laktyushkin <dmytro.laktyushkin@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Swapnil Patel <swapnil.patel@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>