drm/amd/display: Refine i2c frequency calculating sequence

[Why]
In HG mode, vbios didn't call DispController_Init to program NV1x
XTAL_REF_DIV value when ASIC_INIT, but driver read XTAL_REF_DIV
to calculate i2c reference frequency. it cause i2c frequency change
from 100kHz to 200kHz.

[How]
remove get_speed function and calculate reference frequency at
set_speed functiton.

Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>