mesa: replace ONE_DIV_LN2 constant with M_LOG2E

mesa: replace ONE_DIV_LN2 constant with M_LOG2E

1/ln(2) is equivalent to log2(e), so define it as such.

log2(e) = ln(e)/ln(2) = 1/ln(2)

Worst of all, the definitions for M_LOG2E and ONE_DIV_LN2
(right beside each other!) weren't the same.

Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Brian Paul <brianp@vmware.com>