drm/amdgpu: Fix realloc of ptr

Clang static analysis reports this error
amdgpu_debugfs.c:1690:9: warning: 1st function call
  argument is an uninitialized value
  tmp = krealloc_array(tmp, i + 1,
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~

realloc uses tmp, so tmp can not be garbage.
And the return needs to be checked.

Fixes: 5ce5a584 ("drm/amdgpu: add debugfs for reset registers list")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>