Fix intermediate height in convolve_c

- Intermediate height was not correct i.e. when block size is 4 and
  y_step_q4 is 6. In this case intermediate height was
  (4*6) >> 4 = 1 and vertical interpolation needs two source pixels
  plus 7 extra pixels for taps.
- Also if the current output block is 16x16 and we are using 4x upscaling
  we need only 12 rows after horizontal filtering instead of 16.

  Patch Set 2: Intermediate_height updated after CL 66723
               "Fix bug in convolution functions (filter selection)"

Change-Id: I5a1a1bc2ac9d5edb3a6e0818de618bf318fdd589