drm/tests: hdmi: Fix memory leaks in drm_display_mode_from_cea_vic()

drm/tests: hdmi: Fix memory leaks in drm_display_mode_from_cea_vic()

modprobe drm_hdmi_state_helper_test and then rmmod it, the following
memory leak occurs.

The `mode` allocated in drm_mode_duplicate() called by
drm_display_mode_from_cea_vic() is not freed, which cause the memory leak:

	unreferenced object 0xffffff80ccd18100 (size 128):
	  comm "kunit_try_catch", pid 1851, jiffies 4295059695
	  hex dump (first 32 bytes):
	    57 62 00 00 80 02 90 02 f0 02 20 03 00 00 e0 01  Wb........ .....
	    ea 01 ec 01 0d 02 00 00 0a 00 00 00 00 00 00 00  ................
	  backtrace (crc c2f1aa95):
	    [<000000000f10b11b>] kmemleak_alloc+0x34/0x40
	    [<000000001cd4cf73>] __kmalloc_cache_noprof+0x26c/0x2f4
	    [<00000000f1f3cffa>] drm_mode_duplicate+0x44/0x19c
	    [<000000008cbeef13>] drm_display_mode_from_cea_vic+0x88/0x98
	    [<0000000019daaacf>] 0xffffffedc11ae69c
	    [<000000000aad0f85>] kunit_try_run_case+0x13c/0x3ac
	    [<00000000a9210bac>] kunit_generic_run_threadfn_adapter+0x80/0xec
	    [<000000000a0b2e9e>] kthread+0x2e8/0x374
	    [<00000000bd668858>] ret_from_fork+0x10/0x20
	......

Free `mode` by using drm_kunit_display_mode_from_cea_vic()
to fix it.

Cc: stable@vger.kernel.org
Fixes: 4af70f19e559 ("drm/tests: Add RGB Quantization tests")
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030023504.530425-4-ruanjinjie@huawei.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>