efifb: Check efifb_pci_dev before using it

On some platforms like Hyper-V and RPi4 with UEFI firmware, efifb is not
a PCI device.

So make sure efifb_pci_dev is found before using it.

Fixes: a6c0fd3d ("efifb: Ensure graphics device for efifb stays at PCI D0")
BugLink: https://bugs.launchpad.net/bugs/1922403


Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413170508.968148-1-kai.heng.feng@canonical.com