mesa: Fix linker-assigned varying component counting since 8fb1e4a462

mesa: Fix linker-assigned varying component counting since 8fb1e4a462

The goal of that change was to skip counting things that aren't actually
outputs from the VS to the FS.  However, explicit_location isn't set in
the case of linker-assigned locations (the common case), so basically
varying component counting got disabled.  At this stage of the linker,
we've already ensured that var->location is set, so we can just look at
it without worrying.

Fixes i965 assertion failure with the new
piglit glsl-max-varyings --exceed-limits.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51545
Reviewed-by: Brian Paul <brianp@vmware.com>