drm/i915/buddy: tweak 2big check

Otherwise we get -EINVAL, instead of the more useful -E2BIG if the
allocation doesn't fit within the pfn range, like with mappable lmem.
The hugepages selftest, for example, needs this to know if a smaller
size is needed.

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/20220225145502.331818-6-matthew.auld@intel.com