drm/amdgpu: fix the incorrect external id for raven series

This patch fixes the incorrect external id that kernel reports to user mode
driver. Raven2's rev_id is starts from 0x8, so its external id (0x81) should
start from rev_id + 0x79 (0x81 - 0x8). And Raven's rev_id should be 0x21 while
rev_id == 1.

Reported-by: Crystal Jin <Crystal.Jin@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>