drm/xe/migrate: don't overflow max copy size

In the Linux kernel, the following vulnerability has been resolved:

drm/xe/migrate: don't overflow max copy size

With non-page aligned copy, we need to use 4 byte aligned pitch, however
the size itself might still be close to our maximum of ~8M, and so the
dimensions of the copy can easily exceed the S16_MAX limit of the copy
command leading to the following assert:

xe 0000:03:00.0: [drm] Assertion `size / pitch <= ((s16)(((u16)~0U) >> 1))` failed!
platform: BATTLEMAGE subplatform: 1
graphics: Xe2_HPG 20.01 step A0
media: Xe2_HPM 13.01 step A1
tile: 0 VRAM 10.0 GiB
GT: 0 type 1

WARNING: CPU: 23 PID: 10605 at drivers/gpu/drm/xe/xe_migrate.c:673 emit_copy+0x4b5/0x4e0 [xe]

To fix this account for the pitch when calculating the number of current
bytes to copy.

(cherry picked from commit 8c2d61e0e916e077fda7e7b8e67f25ffe0f361fc)