Sunspark
The drivers for graphics devices in the Linux kernel can be found in the drivers/gpu/drm directory. These drivers are included with the kernel itself, and some of them continue to receive updates and improvements over time.
Mesa, on the other hand, is an implementation of graphics APIs such as OpenGL and Vulkan, and it operates at a higher level than the device drivers themselves. However, Mesa includes what are known as DRI drivers, but these are not device drivers in the traditional sense and are part of the Direct Rendering Infrastructure (DRI). These DRI drivers interface with the actual device drivers in the kernel.
In simple terms, Mesa is a way to make hardware acceleration faster by shortening the path from the application to the kernel's graphics device drivers.
P.S. Based on my experience of "digging" into the kernel, although I might be missing something and there might have been something like that in the past.