drm/i915: avoid potential uninitialized variable use

One of the recent changes introduced a warning about
undefined behavior in the sanity checking:

drivers/gpu/drm/i915/intel_ddi.c: In function 'intel_ddi_hdmi_level':
drivers/gpu/drm/i915/intel_ddi.c:654:6: error: 'n_hdmi_entries' may be used uninitialized in this function [-Werror=maybe-uninitialized]

It seems that the new cnl specific get_buf_trans functions
can return uninitialized data if the voltage level is set
to an unexpected value. This changes the code to always return
'1' in that error case, which seems like the safest choice
as we use one less than the number as an array index later on.

Fixes: cc9cabfd ("drm/i915/cnl: Move voltage check into ddi buf trans functions.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[danvet: shut up gcc comment added.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171005120835.437022-1-arnd@arndb.de