frontends/va: Fix some small coverity issues

* 1547216 Operands don't affect result
  - PRESET_MODE_HIGH_QUALITY is maximum value
* 1255678 Dereference after null check
  - no need to check against NULL because the data can
    only be NULL when the array is empty (size == 0)
* 1619397 Unsigned compared against 0
  - check >= 0 is always true in vlVaAddRawHeader
*  1468885 Dereference null return value
  - add NULL check for buffers in vlVaRenderPicture

Acked-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31622>

src/gallium/frontends/va/picture.c

@@ -218,8 +218,7 @@ vlVaHandleVAEncMiscParameterTypeQualityLevel(struct pipe_enc_quality_modes *p, v
          p->pre_encode_mode = PREENCODING_MODE_DEFAULT;
          p->vbaq_mode = VBAQ_AUTO;
       } else {
-         p->preset_mode = in->preset_mode > PRESET_MODE_HIGH_QUALITY
-            ? PRESET_MODE_HIGH_QUALITY : in->preset_mode;
+         p->preset_mode = in->preset_mode;
          p->pre_encode_mode = in->pre_encode_mode;
          p->vbaq_mode = in->vbaq_mode;
       }
@@ -1004,6 +1003,10 @@ vlVaRenderPicture(VADriverContextP ctx, VAContextID context_id, VABufferID *buff
 
    for (i = 0; i < num_buffers && vaStatus == VA_STATUS_SUCCESS; ++i) {
       vlVaBuffer *buf = handle_table_get(drv->htab, buffers[i]);
+      if (!buf) {
+         mtx_unlock(&drv->mutex);
+         return VA_STATUS_ERROR_INVALID_BUFFER;
+      }
 
       switch (buf->type) {
       case VAPictureParameterBufferType:
@@ -1402,7 +1405,7 @@ vlVaAddRawHeader(struct util_dynarray *headers, uint8_t type, uint32_t size,
       memcpy(header.buffer, buf, emulation_bytes_start);
       for (uint32_t i = emulation_bytes_start; i < size; i++) {
          uint8_t byte = buf[i];
-         if (num_zeros >= 2 && byte >= 0x00 && byte <= 0x03) {
+         if (num_zeros >= 2 && byte <= 0x03) {
             header.buffer[pos++] = 0x03;
             num_zeros = 0;
          }

src/gallium/frontends/va/surface.c

@@ -345,9 +345,6 @@ vlVaPutSubpictures(vlVaSurface *surf, vlVaDriver *drv,
    vlVaSubpicture *sub;
    int i;
 
-   if (!(surf->subpics.data || surf->subpics.size))
-      return VA_STATUS_SUCCESS;
-
    for (i = 0; i < surf->subpics.size/sizeof(vlVaSubpicture *); i++) {
       struct pipe_blend_state blend;
       void *blend_state;