videomode: remove timing_entry_index

Display timing's fields have minimum, typical and maximum values. These
can be accessed by using timing_entry_index enum, and
display_timing_get_value() function.

There's no real need for this extra complexity. The values can be
accessed more easily by just using the min/typ/max fields.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>