Add a dummy entry to CEA/HDMI mode tables so they can be indexed
directly using the VIC, avoiding a +1/-1 dance here and there. This adds
clarity to the error checking for various functions that return the VIC
on success and zero on failure; we can now explicitly check for 0
instead of just subtracting one from an unsigned type.

Also add drm_valid_cea_vic() and drm_valid_hdmi_vic() helpers for
checking valid VICs.

v2: add drm_valid_cea_vic and drm_valid_hdmi_vic helpers (Ville)
    use { } instead of { 0 } for initializing the dummy modes

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452252111-6439-1-git-send-email-jani.nikula@intel.comSigned-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>