drm/amd/display: Remove unnecessary DC FP guard

drm/amd/display: Remove unnecessary DC FP guard

[Why & How]
For dcn2x_fast_validate_bw(), not only populate_dml_pipes needs FP guard
but also dml_get_voltage_level().

Remove unnecessary DC_FP_START/DC_FP_END guard in dcn20_fast_validate_bw
and dcn21_fast_validate_bw. FP guard is already there before calling
dcn2x_validate_bandwidth_fp().

Reviewed-by: ChiaHsuan (Tom) Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>