drm/i915/xehp: Support platforms with CCS engines but no RCS

In the past we've always assumed that an RCS engine is present on every
platform.  However now that we have compute engines there may be
platforms that have CCS engines but no RCS, or platforms that are
designed to have both, but have the RCS engine fused off.

Various engine-centric initialization that only needs to be done a
single time for the group of RCS+CCS engines can't rely on being setup
with the RCS now; instead we add a I915_ENGINE_FIRST_RENDER_COMPUTE flag
that will be assigned to a single engine in the group; whichever engine
has this flag will be responsible for some of the general setup
(RCU_MODE programming, initialization of certain workarounds, etc.).

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303223435.2793124-1-matthew.d.roper@intel.com