nv50: report 15 max inputs for fragment programs

First off, nv50_program only has 16 in/out varyings. However reporting
16 makes 'm' become 68 in nv50_fp_linkage_validate with the
varying-packing-simple piglit test. (Subverting the assert makes it
compile but fail.) With this patch, varying-packing-simple passes.

See: https://bugs.freedesktop.org/show_bug.cgi?id=69155

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "9.2 10.0" <mesa-stable@lists.freedesktop.org>

src/gallium/drivers/nouveau/nv50/nv50_screen.c

@@ -226,7 +226,7 @@ nv50_screen_get_shader_param(struct pipe_screen *pscreen, unsigned shader,
    case PIPE_SHADER_CAP_MAX_INPUTS:
       if (shader == PIPE_SHADER_VERTEX)
          return 32;
-      return 0x300 / 16;
+      return 15;
    case PIPE_SHADER_CAP_MAX_CONSTS:
       return 65536 / 16;
    case PIPE_SHADER_CAP_MAX_CONST_BUFFERS: