drm/vmwgfx: fix typo of sizeof argument

Since size of 'header' pointer and '*header' structure is equal on 64-bit
machines issue probably didn't cause any wrong behavior. But anyway,
fixing typo is required.

Fixes: 7a73ba74

 ("drm/vmwgfx: Use TTM handles instead of SIDs as user-space surface handles.")
Co-developed-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230905100203.1716731-1-konstantin.meskhidze@huawei.com