drm/amd/display: Add FPU wrappers to dcn21_validate_bandwidth()

dcn21_validate_bandwidth() calls functions that use floating point math.
On my machine this sometimes results in simd exceptions when there are
other FPU users such as KVM virtual machines running. The screen freezes
completely in this case.

Wrapping the function with DC_FP_START()/DC_FP_END() seems to solve the
problem. This mirrors the approach used for dcn20_validate_bandwidth.

Tested on a AMD Ryzen 7 PRO 4750U (Renoir).

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=206987
Signed-off-by: Jan Kokemüller <jan.kokemueller@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org