drm/gma500: Add NULL check for pci_gfx_root in mid_get_vbt_data()

Since pci_get_domain_bus_and_slot() can return NULL, add NULL check for
pci_gfx_root in the mid_get_vbt_data().

This change is similar to the checks implemented in mid_get_fuse_settings()
and mid_get_pci_revID(), which were introduced by commit 0cecdd81
("gma500: Final enables for Oaktrail") as "additional minor
bulletproofing".

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: f910b411 ("gma500: Add the glue to the various BIOS and firmware interfaces")
Signed-off-by: Ivan Abramov <i.abramov@mt-integration.ru>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306112046.17144-1-i.abramov@mt-integration.ru