drm: Implement drm_need_swiotlb() in drm_cache.c

The function is declared in drm_cache.h. I also removed the curly
braces from the for loop to adhere to kernel coding style.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112081035.6882-3-tzimmermann@suse.de