fbcon: move more common code into fb_open()

fbcon: move more common code into fb_open()

No idea why con2fb_acquire_newinfo() initializes much less than
fbcon_startup(), but so be it. From a quick look most of the
un-initialized stuff should be fairly harmless, but who knows.

Note that the error handling for the con2fb_acquire_newinfo() failure
case was very strange: Callers updated con2fb_map to the new value
before calling this function, but upon error con2fb_acquire_newinfo
reset it to the old value. Since I removed the call to fbcon_release
anyway that strange error path was sticking out like a sore thumb,
hence I removed it. Which also allows us to remove the oldidx
parameter from that function.

v2: Explain what's going on with oldidx and error paths (Sam)

v3: Drop unused variable (0day)

v4: Rebased over bisect fix in previous patch, unchagend end result.

Acked-by: Sam Ravnborg <[email protected]> (v2)
Acked-by: Thomas Zimmermann <[email protected]>
Cc: kernel test robot <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Claudio Suarez <[email protected]>
Cc: Du Cheng <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]