drm/i915: Fix typecheck macro in GT_TRACE

typecheck() macro creates an huge stack size causing
issues with static analysis with coverity, addressing
this with creating a local pointer.

Fixes: 639f2f24

 ("drm/i915: Introduce new macros for tracing")
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216185332.83289-1-venkata.s.dhanalakota@intel.com