x86/bugs: Fix GDS mitigation selecting when mitigation is off

The current GDS mitigation logic incorrectly returns early when the
attack vector mitigation is turned off, which leads to two problems:

1. CPUs without ARCH_CAP_GDS_CTRL support are incorrectly marked with
   GDS_MITIGATION_OFF when they should be marked as
   GDS_MITIGATION_UCODE_NEEDED.

2. The mitigation state checks and locking verification that follow are
   skipped, which means:
   - fail to detect if the mitigation was locked
   - miss the warning when trying to disable a locked mitigation

Remove the early return to ensure proper mitigation state handling. This
allows:
- Proper mitigation classification for non-ARCH_CAP_GDS_CTRL CPUs
- Complete mitigation state verification

This also addresses the failed MSR 0x123 write attempt at boot on
non-ARCH_CAP_GDS_CTRL CPUs:

  unchecked MSR access error: WRMSR to 0x123 (tried to write 0x0000000000000010) at rIP: ... (update_gds_msr)
  Call Trace:
   identify_secondary_cpu
   start_secondary
   common_startup_64
   WARNING: CPU: 1 PID: 0 at arch/x86/kernel/cpu/bugs.c:1053 update_gds_msr

  [ bp: Massage, zap superfluous braces. ]

Fixes: 8c7261ab ("x86/bugs: Add attack vector controls for GDS")
Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://lore.kernel.org/20250819023356.2012-1-lirongqing@baidu.com