drm/amdgpu: Fix a typo in the include header guard of 'navi12_ip_offset.h'

'_navi10_ip_offset_HEADER' is already used in 'navi10_ip_offset.h', so use
'_navi12_ip_offset_HEADER' instead here.

Reviewed-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>