drm/vc4: crtc: Move assigned_channel to a variable

We access multiple times the vc4_crtc_state->assigned_channel variable
in the vc4_crtc_get_scanout_position() function, so let's store it in a
local variable.

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240621152055.4180873-14-dave.stevenson@raspberrypi.com


Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>