st/mesa: Use the texture_subdata memcpy path for non-busy PBO sources.

Civilization VI wants to provide some data to a vertex shader via a
texture buffer.  Unfortunately, the OpenGL port doesn't actually use
texture buffers, and instead does some interesting things:

1. glMapBufferRange with GL_MAP_INVALIDATE_BUFFER_BIT | GL_WRITE_BIT,
   causing the buffer's contents to be dropped and it to be CPU idle.
2. memcpy some data into the buffer.
3. Bind that buffer as a PBO.
4. glTexSubImage2D from the PBO into a 4096xN rectangle texture.
5. Have the shader texelFetch with / and % 4096 coordinate munging.

During the glTexSubImage operation, we see that the source data is
in a PBO and queue a blit.  But in this case the buffer was newly
populated on the CPU and is still idle, so we can simply map it for
reading and upload directly from there.

Eliminates a bunch of unnecessary GPU blits, which seemed to be
taking around 6ms when measured in isolation.
22 jobs for !1771 with pbo-texsubimage in 44 minutes and 22 seconds (queued for 48 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #536650
debian

00:00:21

 
  Build+Test
passed #536659
meson-arm64

00:01:28

passed #536658
meson-armhf

00:01:19

passed #536652
meson-clang

00:11:19

passed #536656
meson-clover

00:12:17

passed #536660
meson-i386

00:02:21

passed #536655
meson-main

00:12:56

passed #536651
meson-swr-glvnd

00:09:28

passed #536657
meson-vulkan

00:02:20

passed #536662
scons-llvm

00:04:33

passed #536661
scons-nollvm

00:03:54

passed #536653
scons-swr

00:04:44

passed #536654
scons-win64

00:07:01

 
  Test
passed #536663
test-llvmpipe-gles2 1/4

00:03:51

passed #536664
test-llvmpipe-gles2 2/4

00:03:31

passed #536665
test-llvmpipe-gles2 3/4

00:04:11

passed #536666
test-llvmpipe-gles2 4/4

00:04:48

passed #536667
test-softpipe-gles2 1/4

00:01:42

passed #536668
test-softpipe-gles2 2/4

00:02:09

passed #536669
test-softpipe-gles2 3/4

00:01:44

passed #536670
test-softpipe-gles2 4/4

00:02:12

passed #536671
test-softpipe-gles3-limited

00:03:58