drm/vc4: hdmi: Rename drm_encoder pointer in mode_valid

The mode_valid hook on the encoder uses a pointer to a drm_encoder called
crtc, which is pretty confusing. Let's rename it to encoder to make it
clear what it is.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7fbabab03992efe4a3a3640ac5ee2bb49b1c7338.1599120059.git-series.maxime@cerno.tech