drm/i915/gt: Relocate Gen6 context-specific workaround

CACHE_MODE_0 register should be saved and restored as part
of the context, not during engine reset. Move the related
workaround (RC_OP_FLUSH_ENABLE) from rcs_engine_wa_init() to
gen6_ctx_workarounds_init() for Gen6 platforms. This ensures the WA
is applied during context initialisation.

CM0_STC_EVICT_DISABLE_LRA_SNB is also Gen6-specific, but it does
not stick when applied in context, so it remains in engine init.

BSPEC: 11322

Signed-off-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/f493bab389e51b2faf7c9a439724e9ea9ca04053.1754902406.git.sebastian.brzezinka@intel.com