drm/amdgpu/gmc11: fix logic typo in AGP check

Should be && rather than ||.

Fixes: b2e1cbe6

 ("drm/amdgpu/gmc11: disable AGP on GC 11.5")
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: Mario Limonciello <mario.limonciello@amd.com> # PHX & Navi33
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>