drm/radeon: refactor SI tiling table initialization

Simplify the control flow of si_tiling_mode_table_init() similar to how
it was done in gfx_v7_0.c and gfx_v8_0.c.
Signed-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>