drm/i915: Addition wrapper for fixed16.16 operation

This patch introduce addition wrapper for fixed point 16.16 operations.
Which will be used by later patches to avoid direct member variables
access of fixed_16_16_t structure.

add_fixed16 : takes 2 fixed_16_16_t variable & returns fixed_16_16_t
add_fixed16_u32 : takes fixed_16_16_t & u32 variable & returns fixed_16_16_t

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170705143154.32132-5-mahesh1.kumar@intel.com