gpu: host1x: Fix missing 'host1x_context_device_bus_type'

The drivers/gpu/host1x/context_bus.c does not include any declaration of
host1x_context_device_bus_type, and after including "context.h" it also
showed that there are two definitions in the kernel because the extern
declaration was missing the const qualifier.

Include linux/host1x_context_bus.h and drop the wrong declaration from
context.h. While at it, also predeclare struct host1x_memory_context.

Fixes the following sparse warning:

    drivers/gpu/host1x/context_bus.c:9:23: warning: symbol 'host1x_context_device_bus_type' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[treding@nvidia.com: minor fixups, reword commit message to reflect changes]
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patch.msgid.link/20260123140512.609167-1-ben.dooks@codethink.co.uk

drivers/gpu/host1x/context.c

@@ -4,6 +4,7 @@
  */
 
 #include <linux/device.h>
+#include <linux/host1x_context_bus.h>
 #include <linux/kref.h>
 #include <linux/of.h>
 #include <linux/of_device.h>

drivers/gpu/host1x/context.h

@@ -12,8 +12,7 @@
 #include <linux/refcount.h>
 
 struct host1x;
-
-extern struct bus_type host1x_context_device_bus_type;
+struct host1x_memory_context;
 
 struct host1x_memory_context_list {
 	struct mutex lock;

drivers/gpu/host1x/context_bus.c

@@ -4,6 +4,7 @@
  */
 
 #include <linux/device.h>
+#include <linux/host1x_context_bus.h>
 #include <linux/of.h>
 
 const struct bus_type host1x_context_device_bus_type = {