drm/amd/display: Fix for outbox1 ring buffer typecasting issue

[WHY]
Compiler warning "pointer to integer of different size" reported on
outbox1 ring buffer address typecasting.

Reported-by: kernel test robot <[e-mail]>

[HOW]
Fixed the issue by typecasting with character pointer.

Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Meenakshikumar Somasundaram <[e-mail]>
Reviewed-by: Jun Lei <[e-mail]>
Acked-by: Solomon Chiu <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dmub/src/dmub_srv.c

···
 
 		// Initialize outbox1 ring buffer
 		rb_params.ctx = dmub;
-		rb_params.base_address = (void *) ((uint64_t) (mail_fb->cpu_addr) + DMUB_RB_SIZE);
+		rb_params.base_address = (void *) ((uint8_t *) (mail_fb->cpu_addr) + DMUB_RB_SIZE);
 		rb_params.capacity = DMUB_RB_SIZE;
 		dmub_rb_init(&dmub->outbox1_rb, &rb_params);