drm/amdgpu/display: use GFP_ATOMIC in dcn20_validate_bandwidth_internal

GFP_KERNEL may and will sleep, and this is being executed in
a non-preemptible context; this will mess things up since it's
called inbetween DC_FP_START/END, and rescheduling will result
in the DC_FP_END later being called in a different context (or
just crashing if any floating point/vector registers/instructions
are used after the call is resumed in a different context).

Signed-off-by: Daniel Kolesa <daniel@octaforge.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>