drm/vc4: Correct generation check in vc4_hvs_lut_load

drm/vc4: Correct generation check in vc4_hvs_lut_load

Commit 24c5ed3ddf27 ("drm/vc4: Introduce generation number enum")
incorrectly swapped a check of hvs->vc4->is_vc5 to
hvs->vc4->gen == VC4_GEN_4 in vc4_hvs_lut_load, hence breaking
loading the gamma look up table on Pi0-3.

Correct that conditional.

Fixes: 24c5ed3ddf27 ("drm/vc4: Introduce generation number enum")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Closes: https://lore.kernel.org/dri-devel/37051126-3921-4afe-a936-5f828bff5752@samsung.com/
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241008-drm-vc4-fixes-v1-3-9d0396ca9f42@raspberrypi.com
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>