drm/i915: Remove intel_limit_t typedef

The coding style documentation says the following about typedefs:

"In general, a pointer, or a struct that has elements that can
reasonably be directly accessed should _never_ be a typedef."

intel_limit_t falls in that category, so just use "struct intel_limit"
instead.
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462353119-9738-3-git-send-email-ander.conselvan.de.oliveira@intel.com