Add a workaround for machines where the framebuffer size reported by the

Add a workaround for machines where the framebuffer size reported by the
hardware is incorrect.  In this case, make sure the amount of "stolen"
memory is at least as large as the EFI framebuffer such that the
driver doesn't use this memory until we've switched to the framebuffer
allocated by the amdgpu(4) driver.

Needs further investigation why the size reported by the hardware is
incorrect.

Tested by djm@
ok jsg@, deraadt@