svga: clarify sys value -> input register mapping

We translate TGSI system value registers to VGPU10 input registers.
Add a comment and set file = TGSI_FILE_INPUT.  That's not stricly
necessary since we map both TGSI_FILE_INPUT and TGSI_FILE_SYSTEM_VALUE
to VGPU10_OPERAND_TYPE_INPUT, but this makes the code a bit more
understandable.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>

src/gallium/drivers/svga/svga_tgsi_vgpu10.c

@@ -1074,7 +1074,9 @@ emit_src_register(struct svga_shader_emitter_v10 *emit,
          }
       }
       else if (file == TGSI_FILE_SYSTEM_VALUE) {
+         /* Map the TGSI system value to a VGPU10 input register */
          assert(index < ARRAY_SIZE(emit->system_value_indexes));
+         file = TGSI_FILE_INPUT;
          index = emit->system_value_indexes[index];
       }
    }