x86/mtrr: Check if fixed-range MTRRs exist in mtrr_save_fixed_ranges()

When suspending, save_processor_state() calls mtrr_save_fixed_ranges()
to save fixed-range MTRRs.

On platforms without fixed-range MTRRs like the ACRN hypervisor which
has removed fixed-range MTRR emulation, accessing these MSRs will
trigger an unchecked MSR access error. Make sure fixed-range MTRRs are
supported before access to prevent such error.

Since mtrr_state.have_fixed is only set when MTRRs are present and
enabled, checking the CPU feature flag in mtrr_save_fixed_ranges() is
unnecessary.

Fixes: 3ebad590 ("[PATCH] x86: Save and restore the fixed-range MTRRs of the BSP when suspending")
Signed-off-by: Jiaqing Zhao <jiaqing.zhao@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250509170633.3411169-2-jiaqing.zhao@linux.intel.com