drm/i915/bios: move debug logging about VBT source to intel_parse_bios()

The decision about which source will be used for VBT is done in
intel_parse_bios(), not in the VBT validation function. Make the VBT
validation function strictly about validation, and move the debug
logging to where it logically belongs.

Also split the logging about where the valid VBT was found and what the
signature is. This will make even more sense in the future when the
validation for ACPI OpRegion based VBT takes place at OpRegion setup
time.

v2: Split logging about VBT signature and BDB version.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178092-27148-1-git-send-email-jani.nikula@intel.com