drm/amd/powerplay: Remove unneeded cast from memory allocation

Remove casting the values returned by memory allocation function.

Coccinelle emits WARNING:

./drivers/gpu/drm/amd/powerplay/hwmgr/vega20_processpptables.c:893:37-46: WARNING: casting value returned by memory allocation function to (PPTable_t *) is useless.

Signed-off-by: Li Heng <liheng40@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>