i965/fs: Fix fs_inst::regs_read() for sources in the ATTR file.

Otherwise it would crash on Gen8 with scalar VS.  The issue can easily
be reproduced with the following patch, but I don't see any reason why
it wouldn't be possible to end up with an ATTR argument here even
without it.

CC: mesa-stable@lists.freedesktop.org
Reviewed-by: Connor Abbott <connor.w.abbott@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -808,6 +808,7 @@ fs_inst::regs_read(int arg) const
    case IMM:
       return 1;
    case GRF:
+   case ATTR:
    case HW_REG:
       return DIV_ROUND_UP(components_read(arg) *
                           src[arg].component_size(exec_size),