drm/i915/guc: Keep GuC interrupts enabled when using GuC

The GuC log contains a separate space used for crash dump.
We even get a separate notification for it. While we're not handling
crash differently yet, it makes sense to decouple the two right now to
simplify the following patches.

v2: Move guc_log_flush_irq_disable up to avoid movement in following
    patches (Sagar).
v3: s/guc_log_flush_irq_*/guc_flush_log_msg_*, rebase after mass rename

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com> (v2)
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180319095348.9716-1-michal.winiarski@intel.com