i915: Adjust line size limits

i915: Adjust line size limits

The hardware can draw lines 0.5 to 7.5 pixels wide. Adjust the limits
to 1.0-7.0. The old limits seems to be from the era when i915 and i965
were sharing this code.

Not really sure if 1.0-7.0 is correct. Maybe it could be 0.5.7.5 as
those are the hw limits, or maybe some combination of the two?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>