drm/i915: Fixed point fixed16 wrapper cleanup

As per suggestion from Jani, cleanup the code. Cleanup includes
 - Instead of left shifting & check, compare with U32/16_MAX
 - Use typecast instead of clamp_t

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170817134529.2839-2-mahesh1.kumar@intel.com