include/linux/io-mapping.h: remove fallback for writecombine

The fallback was introduced in commit 80c33624 ("io-mapping: Fixup
for different names of writecombine") to fix the build on microblaze.

5 years later, it seems all archs now provide a pgprot_writecombine(),
so just remove the other possible fallbacks.  For microblaze,
pgprot_writecombine() is available since commit 97ccedd7
("microblaze: Provide pgprot_device/writecombine macros for nommu").

This is build-tested on microblaze with a hack to always build
mm/io-mapping.o and without DIYing on an x86-only macro
(_PAGE_CACHE_MASK)

Link: https://lkml.kernel.org/r/20211020204838.1142908-1-lucas.demarchi@intel.comSigned-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/io-mapping.h

@@ -132,13 +132,7 @@ io_mapping_init_wc(struct io_mapping *iomap,
 
 	iomap->base = base;
 	iomap->size = size;
-#if defined(pgprot_noncached_wc) /* archs can't agree on a name ... */
-	iomap->prot = pgprot_noncached_wc(PAGE_KERNEL);
-#elif defined(pgprot_writecombine)
 	iomap->prot = pgprot_writecombine(PAGE_KERNEL);
-#else
-	iomap->prot = pgprot_noncached(PAGE_KERNEL);
-#endif
 
 	return iomap;
 }