drm/i915/gvt: Fixed couple of spellings in the file gtt.c

s/negtive/negative/
s/possilbe/possible/

Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210222081838.30328-1-unixbhaskar@gmail.com