drm/i915/vbt: don't propagate errors from intel_bios_init()

We don't use the error return for anything other than reporting and
logging that there is no VBT. We can pull the logging in the function,
and remove the error status return. Moreover, if we needed the
information for something later on, we'd probably be better off storing
the bit in dev_priv, and using it where it's needed, instead of using
the error return.

While at it, improve the comments.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/438ebbb0d5f0d321c625065b9cc78532a1dab24f.1489152288.git.jani.nikula@intel.com