media: ivtv: add parameter to enable ivtvfb on x86 PAT systems

ivtvfb was previously disabled for x86 PAT-enabled systems
by commit 1bf1735b4780 ("x86/mm/pat, drivers/media/ivtv:
Use arch_phys_wc_add() and require PAT disabled") as a
workaround to abstract MTRR code away from device drivers.

The driver is not easily upgradable to the PAT-aware
ioremap_wc() API since the firmware hides the address
ranges that should be marked write-combined from the driver.
However, since a write-combined cache on the framebuffer
is only a performance enhancement not a requirement for
the framebuffer to function, completely disabling the driver
in this configuration is not necessary.

Add force_pat module parameter and a corresponding kernel
configuration parameter to optionally force initialization
on PAT-enabled x86 systems with a warning about the lack of
write-combined caching, and document the reasons the driver
cannot be easily updated to support wc caching on all systems.

Signed-off-by: Nick French <[email protected]>
Signed-off-by: Hans Verkuil <[email protected]>
[[email protected]: fix typo, split long pr_ lines up]
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

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