drm/amd/display: Update get_pixel_clk_frequency() for DCN4x DCCG DP DTO

drm/amd/display: Update get_pixel_clk_frequency() for DCN4x DCCG DP DTO

[Why & How]
DCN4x ASICs have different DCCG logic for programming DP DTO. The current
get_pixel_clk_frequency_100hz() function does not account for this.

Rename the function to "get_dp_dto_frequency" to more accurately
reflect its intended behaviour. Create a new function that correctly
calculates the target pixel rate for DCN4.x DCCG design and use it.

Reviewed-by: Leo Chen <leo.chen@amd.com>
Signed-off-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>