drm/i915: remove AGP dependency from DRM_I915 kconfig help text

We haven't required AGP since 3e99a6b9

 ("drm/i915: Stop depending
upon CONFIG_AGP_INTEL"). Split/rearrange the paragraphs a bit while at
it.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477041257-8219-1-git-send-email-jani.nikula@intel.com