drm/amd/display: work around fp code being emitted outside of DC_FP_START/END

The dcn20_validate_bandwidth function would have code touching the
incorrect registers emitted outside of the boundaries of the
DC_FP_START/END macros, at least on ppc64le. Work around the
problem by wrapping the whole function instead.

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