x86, xsave: Introduce xstate enable functions

The patch renames xsave_cntxt_init() and __xsave_init() into
xstate_enable_boot_cpu() and xstate_enable() as this names are more
meaningful.

It also removes the duplicate xcr setup for the boot cpu.

Signed-off-by: Robert Richter <robert.richter@amd.com>
LKML-Reference: <1279731838-1522-3-git-send-email-robert.richter@amd.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>