drm/i915: add I915_BO_ALLOC_GPU_ONLY

If the user doesn't require CPU access for the buffer, then
ALLOC_GPU_ONLY should be used, in order to prioritise allocating in the
non-mappable portion of LMEM, on devices with small BAR.

v2(Thomas):
  - The BO_ALLOC_TOPDOWN naming here is poor, since this is pure lies on
    systems that don't even have small BAR. A better name is GPU_ONLY,
    which is accurate regardless of the configuration.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225145502.331818-3-matthew.auld@intel.com