Set correct flags mask when validating buffers.

Set correct flags mask when validating buffers.

The 'mask' value used in the validation operation specifies which of the
'flags' bits are being modified. Buffer validation wants to pass the memory
type and access mode (rwx) to the kernel so that the buffer will be placed
correctly, and so that the right kind of fence will be created (read vs
write). That means we actually want a constant mask for these operations,
and not something computed from the bits coming in. The constant we want is
DRM_BO_MASK_MEM | DRM_BO_FLAG_READ | DRM_BO_FLAG_WRITE | DRM_BO_FLAG_EXE.