i965/aa: fixing anti-aliasing bug for thinnest width lines - GEN6

i965/aa: fixing anti-aliasing bug for thinnest width lines - GEN6

On SNB and IVB hw, for 1 pixel line thickness or less,
the general anti-aliasing algorithm give up - garbage line is generated.
Setting a Line Width of 0.0 specifies the rasterization of
the “thinnest” (one-pixel-wide), non-antialiased lines.
Lines rendered with zero Line Width are rasterized using
Grid Intersection Quantization rules as specified
by bspec section 6.3.12.1 Zero-Width (Cosmetic) Line Rasterization.

v2: Daniel Stone: Fix = used instead of == in an if-statement.
v3: Ian Romanick: Use "._Enabled" flag insteed ".Enabled".
    Add code comments. re-word wrap the commit message.
    Add a complete bugzillia list.
    Improve the hardcoded values to produce better results.
v4: Matt Turner: typo fixes and adjust <= 1.49 to become < 1.5

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=28832
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=9951
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=27007
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60797
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=15006

Acked-by: Chris Forbes <chrisf@ijw.co.nz>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Marius Predut <marius.predut@intel.com>