drm/i915/sdvo: Only set is_lvds if we have a valid fixed mode.

If we have failed to ascertain the fixed mode for the LVDS panel, then
trust the pixel clock ranges reported for the connection when determing
valid modes. This makes intel_sdvo_mode_valid() consistent with
intel_lvds_mode_valid() which is also a no-op is there is no fixed mode
defined. (Since the mode is both validated by SDVO and LVDS, why are
checking against an LVDS fixed mode in SDVO...)

By only defining is_lvds to be true when we actually have an LVDS output
with a fixed mode, we avoid various potential NULL deferences where the
assumption is made that all LVDS outputs have a fixed mode.

References:

  Bug 29449 - [Q35] failure to read EDID/vbios for LVDS, no mode => no output
  https://bugs.freedesktop.org/show_bug.cgi?id=29449



The primary failure in this bug is not finding the EDID and determining
the correct fixed panel mode. However, this patch should fix the
secondary issue of not enabling any of the standard modes for the panel
either.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>