drm/amdgpu: properly handle vbios fake edid sizing

drm/amdgpu: properly handle vbios fake edid sizing

The comment in the vbios structure says:
// = 128 means EDID length is 128 bytes, otherwise the EDID length = ucFakeEDIDLength*128

This fake edid struct has not been used in a long time, so I'm
not sure if there were actually any boards out there with a non-128 byte
EDID, but align the code with the comment.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Reported-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lists.freedesktop.org/archives/amd-gfx/2024-June/109964.html
Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>