platform/x86/amd/pmf: Make source_as_str() as non-static

platform/x86/amd/pmf: Make source_as_str() as non-static

Add amd_pmf prefix to source_as_str() function, so that the function name
does not look generic. As this is a helper function make it as non-static
so that it can be reused across multiple PMF features.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Link: https://lore.kernel.org/r/20231212014705.2017474-10-Shyam-sundar.S-k@amd.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>