devm_kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure. Ensure the allocation was successful
by checking the pointer validity.

Release the id allocated in 'mmdc_pmu_init' when 'devm_kasprintf'
return NULL
Suggested-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Fixes: e76bdfd7 ("ARM: imx: Added perf functionality to mmdc driver")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>