drm/i915/ttm: limit where we apply TTM_PL_FLAG_CONTIGUOUS

We only need this when allocating device local-memory, where this
influences the drm_buddy. Currently there is some funny behaviour where
an "in limbo" system memory object is lacking the relevant placement
flags etc. before we first allocate the ttm_tt, leading to ttm
performing a move when not needed, since the current placement is seen
as not compatible.

Suggested-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: 2ed38cec

 ("drm/i915: opportunistically apply ALLOC_CONTIGIOUS")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220324172143.377104-1-matthew.auld@intel.com