backends/drm: Support hardware cursors on NVidia GPUs

This change enables the use of HW cursors on Nvidia GPUs. The problem
with the current approach is that the cursor plane requires a linear
buffer to be attached on the Nvidia hardware. However, Nvidia GPUs
cannot render to a linear texture, this is why DRM_FORMAT_MOD_LINEAR is
marked as "external only" when the supported modifiers are queried by
eglQueryDmaBufModifiersEXT(). Since the EGL render backend in KWin
cannot find a common DRM format modifier between what the cursor plane
needs and what the GPU can render to, it falls back to the SW cursor
implementation.

With this change, when the EGL renderer detects that a plane only
supports linear buffers but rendering directly to a linear buffer is not
possible, it copies the rendered content into a linear dumb buffer, and
attaches that buffer to the plane instead of using the rendered buffer
directly.