drm/radeon: Use vmalloc_array and vcalloc to simplify code

Use vcalloc() and vmalloc_array() to simplify the functions
radeon_gart_init().

vmalloc_array() is also optimized better, resulting in less instructions
being used.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>