drm/radeon: Validate VBIOS signature in VFCT path

drm/radeon: Validate VBIOS signature in VFCT path

The VFCT path accepted whatever kmemdup() returned without checking
that the copied image is a valid VBIOS. Every other radeon BIOS
fetch path verifies the 0x55 0xaa signature before trusting the
image; the VFCT path is the odd one out.

Check the signature after copying the image and reject it (freeing
the buffer) if it does not match, matching the amdgpu VFCT path
which validates via check_atom_bios().

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patch.msgid.link/20260708193518.702584-6-mario.limonciello@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>