dri_util: Update internal_format to GL_RGB8 for MESA_FORMAT_R8G8B8X8_UNORM

dri_util: Update internal_format to GL_RGB8 for MESA_FORMAT_R8G8B8X8_UNORM

The commit helps to resolve GL_INVALID_OPERATION error returned
during CTS test when Android format RGBX8888 fallback to RGBA8888
and then set color with glTexSubImage2D(format=GL_RGB).

Fixes android.hardware.nativehardware.cts.AHardwareBufferNativeTests:
 #SingleLayer_ColorTest_GpuSampledImageCanBeSampled_R8G8B8X8_UNORM

Cc: <mesa-stable@lists.freedesktop.org>
Fixes: bf576772ab4d ("dri_util: add driImageFormatToSizedInternalGLFormat function")
Signed-off-by: Nataraj Deshpande <nataraj.deshpande@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5034>