Revert "i965: Make the miptree clear color setter take a gl_color_union"

Revert "i965: Make the miptree clear color setter take a gl_color_union"

This reverts commit 1d94aa19877fb702ffacacde28ad7253cce72c97.

The next patch will make depth miptrees use the clear color setter that
was originally being used for color miptrees. Go back to using the
isl_color_value parameter because it's the same type as the
fast_clear_color field used by color and depth miptrees.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>