drm/i915: add query uAPI

There are a number of information that are readable from hardware
registers and that we would like to make accessible to userspace. One
particular example is the topology of the execution units (how are
execution units grouped in subslices and slices and also which ones
have been fused off for die recovery).

At the moment the GET_PARAM ioctl covers some basic needs, but
generally is only able to return a single value for each defined
parameter. This is a bit problematic with topology descriptions which
are array/maps of available units.

This change introduces a new ioctl that can deal with requests to fill
structures of potentially variable lengths. The user is expected fill
a query with length fields set at 0 on the first call, the kernel then
sets the length fields to the their expected values. A second call to
the kernel with length fields at their expected values will trigger a
copy of the data to the pointed memory locations.

The scope of this uAPI is only to provide information to userspace,
not to allow configuration of the device.

v2: Simplify dispatcher code iteration (Tvrtko)
    Tweak uapi drm_i915_query_item structure (Tvrtko)

v3: Rename pad fields into flags (Chris)
    Return error on flags field != 0 (Chris)
    Only copy length back to userspace in drm_i915_query_item (Chris)

v4: Use array of functions instead of switch (Chris)

v5: More comments in uapi (Tvrtko)
    Return query item errors in length field (All)

v6: Tweak uapi comments style to match the coding style (Lionel)

v7: Add i915_query.h (Joonas)

v8: (Lionel) Change the behavior of the item iterator to report
    invalid queries into the query item rather than stopping the
    iteration. This enables userspace applications to query newer
    items on older kernels and only have failure on the items that are
    not supported.

v9: Edit copyright headers (Joonas)

v10: Typos & comments in uapi (Joonas)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306122857.27317-6-lionel.g.landwerlin@intel.com