viafb: kill lcd_panel_id

This patch removes all internal uses of another mostly artificial
value. It does duplicate the information of the maximum resolution and
it is not flexible as only a few resolutions exist. Hence it is better
to remove it and clean the mess up.
No runtime change expected.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>