drm/i915/bios: fold pci rom map/unmap into copy function

We don't need to keep the pci rom mapped during the entire
intel_bios_init() anymore. Move it to the previous copy_vbt() function
and rename it to oprom_get_vbt() since now it's responsible to to all
operations related to get the vbt from the oprom.

v2: fix double __iomem attribute detected by sparse
v3: fix missing unmap on success (Ville)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126225110.8127-3-lucas.demarchi@intel.com