lavapipe: fix reported subpixel precision for lines
We have no reason to report a subpixel precision of 4 for lines; in fact LLVMpipe uses 8 subpixel bits for lines, similar to other primitives. But let's use the pipe-cap for this instead of hard-coding it.
Fixes: 9fbf6b2a ("lavapipe: implement VK_EXT_line_rasterization")
Depends on !12327 (closed) to avoid regressions in results
Edited by Erik Faye-Lund