Documentation/i915: Fix kernel-doc references to moved gem files

The error messages could be more descriptive, but fix these caused by
file moves:

WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -internal
    ./drivers/gpu/drm/i915/i915_gem_shrinker.c' failed with return code 2
WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -function
    User command execution ./drivers/gpu/drm/i915/i915_gem_execbuffer.c'
    failed with return code 1
WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -internal
    ./drivers/gpu/drm/i915/i915_gem_tiling.c' failed with return code 2
WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -function
    buffer object tiling ./drivers/gpu/drm/i915/i915_gem_tiling.c'
    failed with return code 1

Fixes: 10be98a7 ("drm/i915: Move more GEM objects under gem/")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605095657.23601-1-jani.nikula@intel.com