drm/amd/display: get refclk from MICROSECOND_TIME_BASE_DIV HW register

drm/amd/display: get refclk from MICROSECOND_TIME_BASE_DIV HW register

[why]
recent VBIOS dce_infotable reference clock change caused a I2c regression.
instead of relying on vbios, let's get it from HW directly.

Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Reviewed-by: Chris Park <Chris.Park@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>