drm/amdgpu: Fix a bug in checking the result of reserve page

Fix if (ret) --> if (!ret), a bug, for
"retire_page", which caused the kernel to recall
the method with *pos == end of file, and that
bounced back with error. On the first run, we
advanced *pos, but returned 0 back to fs layer,
also a bug.

Fix the logic of the check of the result of
amdgpu_reserve_page_direct()--it is 0 on success,
and non-zero on error, not the other way
around. This patch fixes this bug.

Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Cc: John Clements <john.clements@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>