loader: unload command should reset tg_kernel_supported in gfx_state

While loading kernel, we check if vt/vbe backend support is included in
kernel and set the tg_kernel_supported flag in gfx_state. unload
command needs to reset this flag to allow next load to perform
this check with new kernel.

(cherry picked from commit 9b388ac30375ad4e0259b264a006753edcb2bd3c)

Reported by: jhb
Approved by: re (gjb)