drm/amd/display: No need for Null pointer check before kfree

kfree() & vfree() internally performs NULL check on the pointer handed
to it and take no action if it indeed is NULL. Hence there is no need
for a pre-check of the memory pointer before handing it to
kfree()/vfree().

Issue reported by ifnullfree.cocci Coccinelle semantic patch script.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Deepak R Varma <drv@mailo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>