i965: Use brw_get_line_width() in Gen4-5 SF_STATE code.

i965: Use brw_get_line_width() in Gen4-5 SF_STATE code.

This unifies the Gen4-5 and Gen6+ line width calculations.

I believe it also fixes a bug - we weren't rounding the line width
to the nearest integer.  The GL 4.5 (and GL 2.1) specs "Wide Lines"
section says:

"The actual width of non-antialiased lines is determined by rounding
 the supplied width to the nearest integer, then clamping it to the
 implementation-dependent maximum non-antialiased line width."

We don't need to care about _NEW_MULTISAMPLE here because multisampling
doesn't exist on Gen4-5, so the state shouldn't change.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>