drm/i915: Update execlists tasklet naming

intel_lrc_irq_handler and i915_guc_irq_handler are HW submission related
tasklet functions. Name them with "submission_tasklet" suffix and
remove intel/i915 prefix as they are static. Also rename irq_tasklet
as just tasklet for clarity.

v2: s/_bh/_tasklet (Chris)

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1510839162-25197-2-git-send-email-sagar.a.kamble@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>