drm/i915/selftests: Allocate inode/file dynamically

drm/i915/selftests: Allocate inode/file dynamically

Avoid having too large a stack by creating the fake struct inode/file on
the heap instead.

drivers/gpu/drm/i915/selftests/mock_drm.c: In function 'mock_file':
drivers/gpu/drm/i915/selftests/mock_drm.c:46:1: error: the frame size of 1328 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]
drivers/gpu/drm/i915/selftests/mock_drm.c: In function 'mock_file_free':
drivers/gpu/drm/i915/selftests/mock_drm.c:54:1: error: the frame size of 1312 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]

Reported-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 993c92fedee0 ("drm/i915: Mock the GEM device for self-testing")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20170419094143.16922-2-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>