i965: fix const register count for sandybridge

Sandybridge's PS constant buffer payload size is decided from
push const buffer command, incorrect size would cause wrong data
in payload for position and vertex attributes. This fixes coefficients
for tex2d/tex3d.
(cherry picked from commit 81aae67e58)

src/mesa/drivers/dri/i965/brw_wm_glsl.c

@@ -342,8 +342,8 @@ static void prealloc_reg(struct brw_wm_compile *c)
               }
            }
            /* number of constant regs used (each reg is float[8]) */
-           c->nr_creg = 2 * ((4 * nr_params + 15) / 16);
-           reg_index += c->nr_creg;
+	   c->nr_creg = ALIGN(nr_params, 2) / 2;
+	   reg_index += c->nr_creg;
         }
     }