drm/amdgpu/atom: Work around vbios NULL offset false positive

drm/amdgpu/atom: Work around vbios NULL offset false positive

GCC really does not want to consider NULL (or near-NULL) addresses as
valid, so calculations based off of NULL end up getting range-tracked into
being an offset wthin a 0 byte array. It gets especially mad about this:

                if (vbios_str == NULL)
                        vbios_str += sizeof(BIOS_ATOM_PREFIX) - 1;
	...
        if (vbios_str != NULL && *vbios_str == 0)
                vbios_str++;

It sees this as being "sizeof(BIOS_ATOM_PREFIX) - 1" byte offset from
NULL, when building with -Warray-bounds (and the coming
-fdiagnostic-details flag):

In function 'atom_get_vbios_pn',
    inlined from 'amdgpu_atom_parse' at drivers/gpu/drm/amd/amdgpu/atom.c:1553:2:
drivers/gpu/drm/amd/amdgpu/atom.c:1447:34: error: array subscript 0 is outside array bounds of 'unsigned char[0]' [-Werror=array-bounds=]
 1447 |         if (vbios_str != NULL && *vbios_str == 0)
      |                                  ^~~~~~~~~~
  'amdgpu_atom_parse': events 1-2
 1444 |                 if (vbios_str == NULL)
      |                    ^
      |                    |
      |                    (1) when the condition is evaluated to true
......
 1447 |         if (vbios_str != NULL && *vbios_str == 0)
      |                                  ~~~~~~~~~~
      |                                  |
      |                                  (2) out of array bounds here
In function 'amdgpu_atom_parse':
cc1: note: source object is likely at address zero

As there isn't a sane way to convince it otherwise, hide vbios_str from
GCC's optimizer to avoid the warning so we can get closer to enabling
-Warray-bounds globally.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>