drm/amdgpu: Correct misnamed function in amdgpu_gem.c

The header comment above amdgpu_gem_list_handles_ioctl referenced
drm_amdgpu_gem_list_handles_ioctl. Update the comment to reflect the
actual function identifier to avoid misleading prototype warnings.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c:1106: warning: expecting prototype for drm_amdgpu_gem_list_handles_ioctl(). Prototype was for amdgpu_gem_list_handles_ioctl() instead

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>