Optimizes updates of encoder block ptrs

Precalculated block ptrs do not need updates during encoding.
Set these at init stage.

Moved the allocation of 'mt_current_mb_col' (last encoded MB on each
row) to vp8_alloc_compressor_data(), so that it is correctly
reallocated when frame size is changing.

Change-Id: Idcdaa2d0cf3a7f782b7d888626b7cf22a4ffb5c1