drm: Fix shift operations for drm_fb_helper::drm_target_preferred()

smatch correctly warns:

	drivers/gpu/drm/drm_fb_helper.c:1960 drm_target_preferred() warn: should '1 << i' be a 64 bit type?
	drivers/gpu/drm/drm_fb_helper.c:2001 drm_target_preferred() warn: should '1 << i' be a 64 bit type?

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>