drm/amdgpu: fix amdgpu_ras_mca_query_error_status scope

This commit fixes the compile-time warning below:

 warning: no previous prototype for ‘amdgpu_ras_mca_query_error_status’
 [-Wmissing-prototypes]

Changes since v1:
- As suggested by Alexander Deucher:
  1. Make function static instead of adding prototype.

Signed-off-by: Isabella Basso <isabbasso@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>