drm/xe/pf: Fix EAGAIN sign in pf_migration_consume()

drm/xe/pf: Fix EAGAIN sign in pf_migration_consume()

PTR_ERR() returns a negative value, so comparing against the positive
EAGAIN is always true for ERR_PTR(-EAGAIN), causing pf_migration_consume()
to bail out instead of continuing to the remaining GTs. On multi-GT
platforms this can skip GTs that already have data ready.

Compare against -EAGAIN to match the intent (and the following line
that correctly uses -EAGAIN). While at it, gate PTR_ERR() with
IS_ERR().

v2: add IS_ERR() guard before PTR_ERR(). (Gustavo)

Fixes: 67df4a5cbc58 ("drm/xe/pf: Add data structures and handlers for migration rings")
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patch.msgid.link/20260428201448.3999428-1-shuicheng.lin@intel.com
Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>