lcd: allow overriding lcd_get_size()

Remove the redundant lcd_line_length initialisation which
sneaked in when an earlier version of the patch of commit
6d330719 has been rebased.

Some lcd drivers need to setup lcd_line_length not from the
panel_info parameters but by different means. Make the
lcd_get_size() weak to allow setting lcd_line_length in
a driver specific way.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Stephen Warren <swarren@wwwdotorg.org>