drm/amd/display: Send DTBCLK disable message on first commit

[Why]
Previous patch to allow DTBCLK disable didn't address boot case. Driver
thinks DTBCLK is disabled by default, so we don't send disable message to
PMFW. DTBCLK is then enabled at idle desktop on boot, burning power.

[How]
Set dtbclk_en to true on boot so that disable message is sent during first
commit.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Taimur Hassan <syed.hassan@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>