drm/i915: group i915_drv.h forward declarations together

Group the forward declarations in i915_drv.h together near the top, like
in other header files, and sort.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/67f2ed8a9166daa891aa6d89f93eb629145d1c15.1643896905.git.jani.nikula@intel.com