Rework r300 fragprog avoid using bitfield structure.

Rework r300 fragprog avoid using bitfield structure.

It seems that bitfield structure lead to some strange
problem on 64bits arch, don't want to waste time debugging
strange things like that so converted pfs_reg_t structure
to a GLuint and use good old masking and shifting spell.
(cherry picked from 2a7de9d095d8e60da12b11aaa1efe664b87b11d3 commit)