drm/pagemap: Guard HPAGE_PMD_ORDER use with CONFIG_ARCH_ENABLE_THP_MIGRATION

drm/pagemap: Guard HPAGE_PMD_ORDER use with CONFIG_ARCH_ENABLE_THP_MIGRATION

HPAGE_PMD_SHIFT expands to BUILD_BUG() when CONFIG_PGTABLE_HAS_HUGE_LEAVES
is not set, causing a compile error when both CONFIG_TRANSPARENT_HUGEPAGE
and CONFIG_HUGETLB_PAGE are disabled:

 drivers/gpu/drm/drm_pagemap.c:480:12: error: call to '__compiletime_assert_458'
 declared with 'error' attribute: BUILD_BUG failed
   480 |                         order = HPAGE_PMD_ORDER;
       |                                 ^
 include/linux/huge_mm.h:117:26: note: expanded from macro 'HPAGE_PMD_ORDER'
   117 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
       |                          ^
 include/linux/huge_mm.h:113:28: note: expanded from macro 'HPAGE_PMD_SHIFT'
   113 | #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })

Define DRM_PAGEMAP_PMD_ORDER, which maps to HPAGE_PMD_ORDER when
CONFIG_ARCH_ENABLE_THP_MIGRATION is enabled and to -1 otherwise.

This is safe because all code paths that use DRM_PAGEMAP_PMD_ORDER are
reachable only when CONFIG_ARCH_ENABLE_THP_MIGRATION is enabled.

Fixes: 139ab31aea8a ("drm/pagemap: Correct cpages calculation for migrate_vma_setup")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202607201914.LpAGsbXs-lkp@intel.com/
Cc: Jan Stancek <jstancek@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patch.msgid.link/20260721223244.1102276-1-matthew.brost@intel.com