drm/amdgpu: re-format file header comments

Fix the warning below:

 warning: Cannot understand  * \file amdgpu_ioc32.c
 on line 2 - I thought it was a doc line

Changes since v1:
- As suggested by Alexander Deucher:
  1. Reduce diff to minimum as this DOC section doesn't provide much
     value.
Signed-off-by: Isabella Basso <isabbasso@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_ioc32.c

-/**
+/*
  * \file amdgpu_ioc32.c
  *
  * 32-bit ioctl compatibility routines for the AMDGPU DRM.