drm/amdgpu/display: use GFP_ATOMIC in dcn21_validate_bandwidth_fp()

After fixing nested FPU contexts caused by 41401ac6 we're still seeing
complaints about spurious kernel_fpu_end(). As it turns out this was
already fixed for dcn20 in commit f41ed88c ("drm/amdgpu/display:
use GFP_ATOMIC in dcn20_validate_bandwidth_internal") but never moved
forward to dcn21.

Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org