drm/i915: Fix incorrect error handling in shmem_pwrite()

drm/i915: Fix incorrect error handling in shmem_pwrite()

shmem_pwrite() currently checks for short writes before negative error
codes, which can overwrite real errors (e.g., -EFBIG) with -EIO.
Reorder the checks to return negative errors first, then handle short
writes.

Signed-off-by: Taotao Chen <chentaotao@didiglobal.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/20250822030651.28099-2-chentaotao@didiglobal.com