drm/i915/cmdparser: Check reg_table_count before derefencing.

The find_reg function was assuming that there is always at least one table in
reg_tables. It is not always true.

In case of VCS or VECS, the reg_tables is NULL and reg_table_count is 0,
implying that no register-accessing commands are allowed. However, the command
tables include commands such as MI_STORE_REGISTER_MEM. When trying to check
such command, the find_reg would dereference NULL pointer.

Now it will just return NULL meaning that the register was not found and the
command will be rejected.

Fixes: 76ff480e ("drm/i915/cmdparser: Use binary search for faster register lookup")
Signed-off-by: Michal Srb <msrb@suse.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205142916.27092-2-msrb@suse.com
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205160438.3267-1-chris@chris-wilson.co.uk
register lookup")

drivers/gpu/drm/i915/i915_cmd_parser.c

@@ -1032,7 +1032,7 @@ find_reg(const struct intel_engine_cs *engine, bool is_master, u32 addr)
 	const struct drm_i915_reg_table *table = engine->reg_tables;
 	int count = engine->reg_table_count;
 
-	do {
+	for (; count > 0; ++table, --count) {
 		if (!table->master || is_master) {
 			const struct drm_i915_reg_descriptor *reg;
 
@@ -1040,7 +1040,7 @@ find_reg(const struct intel_engine_cs *engine, bool is_master, u32 addr)
 			if (reg != NULL)
 				return reg;
 		}
-	} while (table++, --count);
+	}
 
 	return NULL;
 }