drm/msm: msm_iommu.c: fix all kernel-doc warnings

drm/msm: msm_iommu.c: fix all kernel-doc warnings

Correct or add kernel-doc comments to eliminate all warnings:

Warning: ../drivers/gpu/drm/msm/msm_iommu.c:381 expecting prototype for
 alloc_pt(). Prototype was for msm_iommu_pagetable_alloc_pt() instead
Warning: ../drivers/gpu/drm/msm/msm_iommu.c:426 expecting prototype for
 free_pt(). Prototype was for msm_iommu_pagetable_free_pt() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/695675/
Link: https://lore.kernel.org/r/20251219184638.1813181-20-rdunlap@infradead.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>