drm/amdgpu: Fix missing prototype warning

Fix a missing prototype warning for function
amdgpu_info_ioctl(),

drivers/gpu/drm/amd/amdgpu/amdgpu_kms.c:482:5: warning: no previous prototype for 'amdgpu_info_ioctl' [-Wmissing-prototypes]

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201110051548.685725-1-luben.tuikov@amd.com