thermal: core: Delay exposing sysfs interface

There's a race between initializing the governor and userspace accessing
the sysfs interface. From time to time the Intel graphics CI shows this
signature:

	<1>[] #PF: error_code(0x0000) - not-present page
	<6>[] PGD 0 P4D 0
	<4>[] Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
	<4>[] CPU: 3 UID: 0 PID: 562 Comm: thermald Not tainted 6.14.0-rc4-CI_DRM_16208-g7e37396f86d8+ #1
	<4>[] Hardware name: Intel Corporation Twin Lake Client Platform/AlderLake-N LP5 RVP, BIOS TWLNFWI1.R00.5222.A01.2405290634 05/29/2024
	<4>[] RIP: 0010:policy_show+0x1a/0x40

thermald tries to read the policy file between the sysfs files being
created and the governor set by thermal_set_governor(), which causes the
NULL pointer dereference.

Similarly to the hwmon interface, delay exposing the sysfs files to when
the governor is already set.

[ topic/core-for-CI:
  # Closes: #13655
  # Link: https://lore.kernel.org/all/CAJZ5v0gYSKWJySx+KC7=zhu4xw3eNrn-LRPrMViz2KSYvJ6dKA@mail.gmail.com/
  "Applied as 6.15 material, thanks!" - Rafael J. Wysocki ]
References: #13932


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>