drm/amd/display: Fixed panic during seamless boot.

[why]
get_pixel_clk_frequency_100hz is undefined in clock_source_funcs.

[how]
set function pointer: ".get_pixel_clk_frequency_100hz = get_pixel_clk_frequency_100hz"

Signed-off-by: David Galiffi <David.Galiffi@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>