drm/i915/gvt: fix off-by-one comparison of ring_id

The ring_id maximum boundary is being compared using the > operator
instead of >=, leading to an off-by-one error and an out of bounds
write into array vgpu->hws_pga[].  Fix this by simply using the
correct comparison operator. Also re-work another comparison that
uses the comparison > I915_NUM_ENGINES - 1 to use the >= idiom using
to keep this consistent in this code.

Detected by CoverityScan, CID#1462404 ("Out-of-bounds write")

Fixes: a2ae95af ("drm/i915/gvt: update CSB and CSB write pointer in virtual HWSP")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/handlers.c

@@ -1398,7 +1398,7 @@ static int hws_pga_write(struct intel_vgpu *vgpu, unsigned int offset,
 	 * update the VM CSB status correctly. Here listed registers can
 	 * support BDW, SKL or other platforms with same HWSP registers.
 	 */
-	if (unlikely(ring_id < 0 || ring_id > I915_NUM_ENGINES)) {
+	if (unlikely(ring_id < 0 || ring_id >= I915_NUM_ENGINES)) {
 		gvt_vgpu_err("VM(%d) access unknown hardware status page register:0x%x\n",
 			     vgpu->id, offset);
 		return -EINVAL;
@@ -1507,7 +1507,7 @@ static int elsp_mmio_write(struct intel_vgpu *vgpu, unsigned int offset,
 	u32 data = *(u32 *)p_data;
 	int ret = 0;
 
-	if (WARN_ON(ring_id < 0 || ring_id > I915_NUM_ENGINES - 1))
+	if (WARN_ON(ring_id < 0 || ring_id >= I915_NUM_ENGINES))
 		return -EINVAL;
 
 	execlist = &vgpu->submission.execlist[ring_id];