drm/i915: Add braces around the else block in clflush_write32()

According to the kernel coding style, if only one branch of a
conditional statement is a single statement, braces should
still be used in both branches.

Signed-off-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20250718102752.684975-2-sebastian.brzezinka@intel.com