dir-locals.el: Don't set variables for non-programming modes

This limits the style changes to modes inherited from prog-mode. The
main reason to do this is to avoid setting fill-column for people
using Emacs to edit commit messages because 78 characters is too many
to make it wrap properly in git log. Note that makefile-mode also
inherits from prog-mode so the fill column should continue to apply
there.

v2: Apply to all the .dir-locals.el files, not just the one in the
    root directory.

Acked-by: Michel Dänzer <michel.daenzer@amd.com>

.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . nil)
   (tab-width . 8)
   (c-basic-offset . 3)

src/gallium/drivers/freedreno/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . true)
   (tab-width . 4)
   (c-basic-offset . 4)

src/gallium/drivers/r600/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . true)
   (tab-width . 8)
   (c-basic-offset . 8)

src/gallium/drivers/radeon/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . true)
   (tab-width . 8)
   (c-basic-offset . 8)

src/gallium/drivers/radeonsi/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . true)
   (tab-width . 8)
   (c-basic-offset . 8)

src/gallium/drivers/vc4/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . nil)
   (tab-width . 8)
   (c-basic-offset . 8)

src/gallium/drivers/vc4/kernel/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . t)
   (tab-width . 8)
   (c-basic-offset . 8)

src/gallium/winsys/radeon/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . nil)
   (tab-width . 8)
   (c-basic-offset . 4)

src/mesa/drivers/dri/nouveau/.dir-locals.el

@@ -1,4 +1,4 @@
-((nil
+((prog-mode
   (indent-tabs-mode . true)
   (tab-width . 8)
   (c-basic-offset . 8)