drm/i915: Restrict PSMI context load w/a to Haswell GT1

After we found a workaround for a hang on context load, Ben Widawsky
found confirmation that it was for an issue with waking from rc6 and
loading a context image.

The workaround from on high suggests that we should

	I915_WRITE(RING_WAIT_FOR_RC6_EXIT(engine->mmio_base),
		   _MASKED_FIELD(RING_RC6_SEL_WRITE_ADDR_MASK,
				 RING_RC6_SEL_WRITE_ADDR_UPPER_LEFT));

in our rc6 setup for Haswell GT1, but on applying that we find instead
that the machine encounters a GT forcewake error and locks up.

As we are removing HW semaphore usage in the next patch, and the
suggested workaround is no improvement, we need to
decouple the PSMI workaround from HAS_SEMAPHORES to IS_HSW_GT1.

References: 2c550183 ("drm/i915: Disable PSMI sleep messages on all rings around context switches")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181228140736.32606-1-chris@chris-wilson.co.uk