drm/i915: cleanup: use ARRAY_SIZE()

NUM_TV_MODES is the same as ARRAY_SIZE(tv_modes).  In the end, I
decided it was cleaner to remove NUM_TV_MODES and just use
ARRAY_SIZE(tv_modes) through out.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>