meson: Add default DRI drivers for ARM/AArch64

On all Arm architectures (ARMv7 and below as 'arm', ARMv8 and above as
'aarch64'), only build swrast for DRI drivers. The only classic drivers
which could be used are r200 and NV20 cards, which seems unlikely enough
that it shouldn't be the default.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: Javier Jardón <jjardon@gnome.org>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

meson.build

@@ -104,6 +104,8 @@ if _drivers == 'auto'
     # TODO: PPC, Sparc
     if ['x86', 'x86_64'].contains(host_machine.cpu_family())
       _drivers = 'i915,i965,r100,r200,nouveau'
+    elif ['arm', 'aarch64'].contains(host_machine.cpu_family())
+      _drivers = ''
     else
       error('Unknown architecture. Please pass -Ddri-drivers to set driver options. Patches gladly accepted to fix this.')
     endif