llvmpipe: cleanup of generate_unswizzled_blend

llvmpipe: cleanup of generate_unswizzled_blend

Some parameters were used inconsistently, for instance not using
block_width/block_height/block_size for deferring number of pixels
but rather relying on guesses from the number of fragment shaders etc,
so fix this up (no actual change in behavior since the block size stays
fixed). (Though most of the code would work with different block_height,
with three exceptions, one being the hacked r11g11b10 conversions and
twiddle code which only work with block_height 2 not 1, and the last
one being blend vector type not being 128bit wide.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>