i965: Optimize bucket index calculation

i965: Optimize bucket index calculation

Reducing Bucket index calculation to O(1).

This algorithm calculates the index using matrix method.  Assuming
PAGE_SIZE is 4096, matrix arrangement is as below:

          1*4096   2*4096    3*4096    4*4096
          5*4096   6*4096    7*4096    8*4096
          10*4096  12*4096   14*4096   16*4096
          20*4096  24*4096   28*4096   32*4096
           ...      ...       ...       ...
           ...      ...       ...       ...
           ...      ...       ...   max_cache_size

From this matrix its clearly seen that every row follows the below way:

          ...       ...       ...        n
        n+(1/4)n  n+(1/2)n  n+(3/4)n    2n

Row is calculated as log2(size/PAGE_SIZE) Column is calculated as
converting the difference between the elements to fit into power size of
two and indexing it.

Final Index is (row*4)+(col-1)

Tested with Intel Mesa CI.

Improves performance of 3DMark on BXT by 0.705966% +/- 0.229767% (n=20)

v4: Review comments on style and code comments implemented (Ian).
v3: Review comments implemented (Ian).
v2: Review comments implemented (Jason).

Signed-off-by: Aravindan Muthukumar <aravindan.muthukumar@intel.com>
Signed-off-by: Kedar Karanje <kedar.j.karanje@intel.com>
Reviewed-by: Yogesh Marathe <yogesh.marathe@intel.com>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>