drm/i915/display: fix missing null check on allocated dsb object

Currently there is no null check for a failed memory allocation
on the dsb object and without this a null pointer dereference
error can occur. Fix this by adding a null check.

Note: added a drm_err message in keeping with the error message style
in the function.

Addresses-Coverity: ("Dereference null return")
Fixes: afeda4f3 ("drm/i915/dsb: Pre allocate and late cleanup of cmd buffer")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200616114221.73971-1-colin.king@canonical.com

drivers/gpu/drm/i915/display/intel_dsb.c

@@ -271,6 +271,10 @@ void intel_dsb_prepare(struct intel_crtc_state *crtc_state)
 		return;
 
 	dsb = kmalloc(sizeof(*dsb), GFP_KERNEL);
+	if (!dsb) {
+		drm_err(&i915->drm, "DSB object creation failed\n");
+		return;
+	}
 
 	wakeref = intel_runtime_pm_get(&i915->runtime_pm);