drm/i915: Show bios vbt when read from firmware/spi/oprom

Make debugfs vbt only shows valid vbt when read from ACPI opregion.
Make it work when read from firmware/spi/pci oprom cases. In the cases
where VBT needs to be read from spi/pci oprom, take the wakeref to
prevent WARN while reading DE registers during debugfs vbt dump.

v2: Extract getting vbt from different sources to its own function.
    Protect sysfs write with vbt check(Jani)
v3: Fix CI error by probing bios vbt with runtime_pm wakeref
v4: Update commit message and skip waking up runtime while accessing
    vbt from opregion/firmware(Jani)
v5: Skip grabbing unnecessary wakeref(Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240304212331.640424-1-radhakrishna.sripada@intel.com