mesa: Drop index buffer bounds check

mesa: Drop index buffer bounds check

The previous check was insufficient (as it did not take 'indices' into
consideration), and DX10 hardware does not need this check anyway.

Since index_bytes is no longer used, remove it.

On Bay Trail-D using Fedora 20 compile flags (-m64 -O2 -mtune=generic
for 64-bit and -m32 -march=i686 -mtune=atom for 32-bit), affects
Gl32Batch7:

32-bit: Difference at 95.0% confidence 1.66929% +/- 0.230107% (n=40)
64-bit: Difference at 95.0% confidence -1.40848% +/- 0.288038% (n=40)

The regression on 64-bit is odd.  Callgrind says the caller,
validate_DrawElements_common is faster.  Before it says 10,321,920
cycles, and after it says 8,945,664.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>