gallium/vl: Don't support planar RGB as video format

We assume everywhere that RGB is not planar, so sampling
and color space conversions will not work correctly with RGBP.
Drivers can still support RGBP, but processing entrypoint with
shaders doesn't support it.

Fixes: bdb7f36aa8 ("frontends/va: add support for RGBP rt_format")
Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32252>

src/gallium/auxiliary/vl/vl_video_buffer.c

@@ -120,6 +120,9 @@ vl_video_buffer_is_format_supported(struct pipe_screen *screen,
    enum pipe_format resource_formats[VL_NUM_COMPONENTS];
    unsigned i;
 
+   if (entrypoint == PIPE_VIDEO_ENTRYPOINT_PROCESSING && format == PIPE_FORMAT_R8_G8_B8_UNORM)
+      return false;
+
    vl_get_video_buffer_formats(screen, format, resource_formats);
 
    for (i = 0; i < VL_NUM_COMPONENTS; ++i) {