drm/amdgpu: fix retry loop test

This loop will exit with "retry" set to -1 if it fails but the code
checks for if "retry" is zero.  Fix this by changing post-op to a
pre-op.  --retry vs retry--.

Fixes: e01eeffc ("drm/amd/pm: avoid driver getting empty metrics table for the first time")
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>