tgafb: clarify dependencies

The TGA boards were based on the DECchip 21030 PCI graphics accelerator
used mainly for alpha, and existed in a TURBOchannel (TC) version for
the DECstation (MIPS) workstations.

However, the config option for the TGA code is a bit confused, and says

	depends on FB && (ALPHA || TC)

because people didn't really want to enable the option for random PCI
environments, so the "ALPHA" stands in for that case (while the TC case
is then the MIPS DECstation case).

So that config dependency is kind of a mixture of architecture and bus
choices.  But it's incorrect, in that there were non-PCI-based alpha
hardware, and then the driver just causes warnings:

  drivers/video/fbdev/tgafb.c:1532:13: error: ‘tgafb_unregister’ defined but not used [-Werror=unused-function]
   1532 | static void tgafb_unregister(struct device *dev)
        |             ^~~~~~~~~~~~~~~~
  drivers/video/fbdev/tgafb.c:1387:12: error: ‘tgafb_register’ defined but not used [-Werror=unused-function]
   1387 | static int tgafb_register(struct device *dev)
        |            ^~~~~~~~~~~~~~

so let's make the config option dependencies a bit more explict:

	depends on FB
	depends on PCI || TC
	depends on ALPHA || TC

where that first "FB" is the software configuration dependency, the
second "PCI || TC" is the hardware bus dependency, while that final
"ALPHA || TC" dependency is the "don't bother asking except for these
situations.

We could make that third case have "COMPILE_TEST" as an option, and mark
the register/unregister functions as __maybe_unused, but I'm not sure
it's really worth it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>