drm/i915: Match bitmask size to types in intel_fb_initial_config()

smatch complains of:
	drivers/gpu/drm/i915/intel_fbdev.c:403
	intel_fb_initial_config() warn: should '1 << i' be a 64 bit type?
	drivers/gpu/drm/i915/intel_fbdev.c:422 intel_fb_initial_config() warn:
	should '1 << i' be a 64 bit type?
	drivers/gpu/drm/i915/intel_fbdev.c:501 intel_fb_initial_config() warn:
	should '1 << i' be a 64 bit type?

We are prepared to iterate over a u64 but don't limit the number of
connectors we try to configure to a maximum of 64.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1467470166-31717-5-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>