drm/vc4: crtc: Remove the feed_txp tests

Now that the code in vc4_crtc accessing registers is only meant for the
pixelvalve, it doesn't make sense anymore to test whether we're accessing
the TXP or not and we can safely remove those checks.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/c044daba470fcb1cb57e3d34d88f75325b2ebbab.1591882579.git-series.maxime@cerno.tech