accel/ivpu: Allow reading dvfs_mode debugfs file

Make the dvfs_mode read-write to allow checking current mode.
Simplify the dvfs_mode implementation with the DEFINE_DEBUGFS_ATTRIBUTE.

Signed-off-by: Andrzej Kacprowski <Andrzej.Kacprowski@intel.com>
Reviewed-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930195322.461209-15-jacek.lawrynowicz@linux.intel.com


Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>