drm/amd/display: Fix rn audio playback and video playback speed

[WHY]
dprefclk is improperly read due to incorrect units used.
Causes an audio clock to be improperly set, making audio
non-functional and videos play back too fast

[HOW]
Scale dprefclk value from MHz to KHz (multiply by 1000)
to ensure that dprefclk_khz is in correct units

Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>