drm/ttm: add pgprot handling for RISC-V

The RISC-V Svpbmt privileged extension provides support for overriding
page memory coherency attributes, and, along with vendor extensions like
Xtheadmae, supports pgprot_{writecombine,noncached} on RISC-V.

Adapt the codepath that maps ttm_write_combined to pgprot_writecombine
and ttm_noncached to pgprot_noncached to RISC-V, to allow proper page
access attributes.

Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Tested-by: Han Gao <rabenda.cn@gmail.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20251020053523.731353-1-uwu@icenowy.me

Author: Icenowy

drivers/gpu/drm/ttm/ttm_module.c

@@ -74,7 +74,8 @@ pgprot_t ttm_prot_from_caching(enum ttm_caching caching, pgprot_t tmp)
 #endif /* CONFIG_UML */
 #endif /* __i386__ || __x86_64__ */
 #if defined(__ia64__) || defined(__arm__) || defined(__aarch64__) || \
-	defined(__powerpc__) || defined(__mips__) || defined(__loongarch__)
+	defined(__powerpc__) || defined(__mips__) || defined(__loongarch__) || \
+	defined(__riscv)
 	if (caching == ttm_write_combined)
 		tmp = pgprot_writecombine(tmp);
 	else