drm/panfrost: Remove incorrect IS_ERR() check

drm/panfrost: Remove incorrect IS_ERR() check

sg_page_iter_page() doesn't return an error code, so the IS_ERR() check
is wrong and the error path will never be executed. This also allows
simplifying the code to remove the local variable 'page'.

CC: Adrián Larumbe <adrian.larumbe@collabora.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/376713ff-9a4f-4ea3-b097-fb5efb685d95@moroto.mountain
Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Tested-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231020104405.53992-1-steven.price@arm.com