drm/amd/display: Slightly optimize dm_dmub_outbox1_low_irq()

A kzalloc()+memcpy() can be optimized in a single kmemdup().
This saves a few cycles because some memory doesn't need to be zeroed.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>