vk: Only begin subpass if we're continuing a render pass

vk: Only begin subpass if we're continuing a render pass

If VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT is not set in
pBeginInfo->flags, we don't have a render pass or framebuffer. Change
the condition that guard looking up render pass and framebuffer to test
for VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT instead of
VK_COMMAND_BUFFER_LEVEL_SECONDARY.

Fixes all remaining crashes in dEQP-VK.api.command_buffers.*.