drm/amd: Avoid reading the VBIOS part number twice

The VBIOS part number is read both in amdgpu_atom_parse() as well
as in atom_get_vbios_pn() and stored twice in the `struct atom_context`
structure. Remove the first unnecessary read and move the `pr_info`
line from that read into the second.

v2: squash in unused variable removal

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>