r600: Fix maximum line width

There are only 13 bits available to store the line width, hence
it can't be larger than 8191

v2: Add Fixes tag

v3: - Unify value since for all r600 archs (Konstantin Kharlamov)
    - Correct the value the line width value is emitted as a 12.4
      fixed point value of 1/2 line width on r600-r700 and as
      8 * line width on Evergreen and newer.

Fixes: 06bfb2d2
    r600: fork and import gallium/radeon

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Konstantin Kharlamov <hi-angel@yandex.ru>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3286>
(cherry picked from commit e8559ae4)