drm/i915: mark pwrite/pread slowpaths with unlikely

Beside helping the compiler untangle this maze they double-up as
documentation for which parts of the code aren't performance-critical
but just around to keep old (but already dead-slow) userspace from
breaking.
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -298,7 +298,7 @@ shmem_pread_fast(struct page *page, int shmem_page_offset, int page_length,
 	char *vaddr;
 	int ret;
 
-	if (page_do_bit17_swizzling)
+	if (unlikely(page_do_bit17_swizzling))
 		return -EINVAL;
 
 	vaddr = kmap_atomic(page);
@@ -317,7 +317,7 @@ static void
 shmem_clflush_swizzled_range(char *addr, unsigned long length,
 			     bool swizzled)
 {
-	if (swizzled) {
+	if (unlikely(swizzled)) {
 		unsigned long start = (unsigned long) addr;
 		unsigned long end = (unsigned long) addr + length;
 
@@ -629,7 +629,7 @@ shmem_pwrite_fast(struct page *page, int shmem_page_offset, int page_length,
 	char *vaddr;
 	int ret;
 
-	if (page_do_bit17_swizzling)
+	if (unlikely(page_do_bit17_swizzling))
 		return -EINVAL;
 
 	vaddr = kmap_atomic(page);
@@ -660,7 +660,7 @@ shmem_pwrite_slow(struct page *page, int shmem_page_offset, int page_length,
 	int ret;
 
 	vaddr = kmap(page);
-	if (needs_clflush_before || page_do_bit17_swizzling)
+	if (unlikely(needs_clflush_before || page_do_bit17_swizzling))
 		shmem_clflush_swizzled_range(vaddr + shmem_page_offset,
 					     page_length,
 					     page_do_bit17_swizzling);