drm/amd/display: Rename set_mst_bandwidth to align with DP spec

[Why]
The function set_mst_bandwidth is poorly name since it isn't clear what
it does, and it also does not reflect any part of the allocation sequence
described in the DP spec.

[How]
Rename the function set_mst_bandwidth to set_throttled_vcp_size.

Signed-off-by: George Shen <george.shen@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>