drm/i915: Shrink cxsr_latency_table

unsigned long is too wide - use smaller types in
struct cxsr_latency to save 800-something bytes of .rodata.

v2: All data even fits in u16 for even more saving. (Ville Syrjala)
v3: Move bitfields to the end of the struct. (Joonas Lahtinen)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>