drm/radeon: Fix multiple line dereference in 'atom_iio_execute'

Fixes the following:

WARNING: Avoid multiple line dereference - prefer 'ctx->io_attr'
+                           ((ctx->
+                             io_attr >> CU8(base + 2)) & (0xFFFFFFFF >> (32 -

Cc: Guchun Chen <guchun.chen@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>