drm/xe: Use DRM_BUDDY_CONTIGUOUS_ALLOCATION for contiguous allocations

drm/xe: Use DRM_BUDDY_CONTIGUOUS_ALLOCATION for contiguous allocations

The VRAM/stolen memory managers do not currently set
DRM_BUDDY_CONTIGUOUS_ALLOCATION for contiguous allocations. Enabling
this flag activates the buddy allocator's try_harder path, which helps
handle fragmented memory scenarios.

This enables the __alloc_contig_try_harder fallback in the buddy
allocator, allowing contiguous allocation requests to succeed even when
memory is fragmented by combining allocations from both(RHS and LHS)
sides of a large free block.

v2: (Matt B)
- Remove redundant logic for rounding allocation size and trimming when
  TTM_PL_FLAG_CONTIGUOUS is set, since drm_buddy now handles this when
  DRM_BUDDY_CONTIGUOUS_ALLOCATION is enabled

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6713
Suggested-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Sanjay Yadav <sanjay.kumar.yadav@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patch.msgid.link/20260121111416.3104399-2-sanjay.kumar.yadav@intel.com