mesa: remove incorrect (float) cast in mipmap do_row()

mesa: remove incorrect (float) cast in mipmap do_row()

The array holds GLuint values so remove the float cast.
Note, however, that to compute the average of four GLuints we really
want to do (a+b+c+d)/4 but that could overflow.  This change doesn't
address that for now.

NOTE: This is a candidate for the 8.0 branch.

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