simplify upsampler calls: only allow 'bottom' to be NULL

If 'top' was meant to be NULL, then bottom and top can be
swapped. Logic is simpler.

+ fix compilation in non-FANCY_UPSAMPLING mode

Change-Id: I7c62bbb59454017f072c0945d1ff2d24d89286ff

src/dsp/dsp.h

@@ -146,6 +146,8 @@ void VP8DspInit(void);
 
 #define FANCY_UPSAMPLING   // undefined to remove fancy upsampling support
 
+// Convert a pair of y/u/v lines together to the output rgb/a colorspace.
+// bottom_y can be NULL if only one line of output is needed (at top/bottom).
 typedef void (*WebPUpsampleLinePairFunc)(
     const uint8_t* top_y, const uint8_t* bottom_y,
     const uint8_t* top_u, const uint8_t* top_v,