drm/tests/drm_kunit_helpers: Place correct function name in the comment header

drm/tests/drm_kunit_helpers: Place correct function name in the comment header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/tests/drm_kunit_helpers.c:172: warning: expecting prototype for drm_kunit_helper_context_alloc(). Prototype was for drm_kunit_helper_acquire_ctx_alloc() instead

Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230824073710.2677348-10-lee@kernel.org
Link: https://lore.kernel.org/r/20230824073710.2677348-14-lee@kernel.org
[mripard: Squashed the two patches together]
Signed-off-by: Maxime Ripard <mripard@kernel.org>