drm/ast: Remove little-endianism from I/O helpers

Replace one call to ast_io_write16() with two calls to ast_io_write8()
in ast_set_index_reg(). The combined 16-bit-wide write of an index
register and the corresponding data register only works on little-
endian systems. Write both registers independent from each other.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230221155745.27484-2-tzimmermann@suse.de

drivers/gpu/drm/ast/ast_drv.h

@@ -282,7 +282,9 @@ static inline void ast_set_index_reg(struct ast_private *ast,
 				     uint32_t base, uint8_t index,
 				     uint8_t val)
 {
-	ast_io_write16(ast, base, ((u16)val << 8) | index);
+	ast_io_write8(ast, base, index);
+	++base;
+	ast_io_write8(ast, base, val);
 }
 
 void ast_set_index_reg_mask(struct ast_private *ast,